#### beero1000

##### Veteran Member
For those interested (or bored) enough, a new thread to pose/solve math problems.

1. Suppose the roots of the polynomial x5 + 2x4 + 3x3 + 4x2 + 5x + 6 are a, b, c, d, and e, respectively. Find a2+b2+c2+d2+e2. Bonus points: Find the harmonic mean of the roots (i.e. 5 times the reciprocal of the sum of their reciprocals).
2. An irregular octagon is inscribed in a circle. The edge lengths of the octagon are 4,4,4,4,2,2,2,2. Find the area of the octagon. Bonus points: Find the radius of the circle.
3. Is (14n+3)/(21n + 4) a fraction in simplest form for every n?
4. Two unit circles pass through each other's center. Find the area of their intersection. Bonus points: Same question for 3 intersecting circles.

#### ApostateAbe

##### Veteran Member
I have the solution to #1.

x^5 + 2*x^4 + 3*x^3 + 4*x^2 + 5*x + 6 = 0

http://www.mathportal.org/calculators/solving-equations/polynomial-equation-solver.php

a = -1.4918
b = 0.55169+1.25335*i
c = 0.55169-1.25335*i
d = -0.80579+1.2229*i
e = -0.80579-1.2229*i

a^2+b^2+c^2+d^2+e^2=

(-1.4918)^2+(0.55169+1.25335*i)^2+(0.55169-1.25335*i)^2+(-0.80579+1.2229*i)^2+(-0.80579-1.2229*i)^2=

-1.9999552646

approximately equals (could very well be exact solution)

-2

I will solve the others later.

#### beero1000

##### Veteran Member
I have the solution to #1.

x^5 + 2*x^4 + 3*x^3 + 4*x^2 + 5*x + 6 = 0

http://www.mathportal.org/calculators/solving-equations/polynomial-equation-solver.php

a = -1.4918
b = 0.55169+1.25335*i
c = 0.55169-1.25335*i
d = -0.80579+1.2229*i
e = -0.80579-1.2229*i

a^2+b^2+c^2+d^2+e^2=

(-1.4918)^2+(0.55169+1.25335*i)^2+(0.55169-1.25335*i)^2+(-0.80579+1.2229*i)^2+(-0.80579-1.2229*i)^2=

-1.9999552646

approximately equals (could very well be exact solution)

-2

I will solve the others later.

The exact value is indeed -2. The main question is ... can you prove it?

#### ApostateAbe

##### Veteran Member
I am an empiricist and I care not for damn proofs!

#### beero1000

##### Veteran Member
I am an empiricist and I care not for damn proofs!

Well then, I guess you could empirically determine the sum of the squares of the roots of the polynomial equation a5x5+a4x4+a3x3+a2x2+a1x+a0=0.

#### Bomb#20

##### Contributor
2. An irregular octagon is inscribed in a circle. The edge lengths of the octagon are 4,4,4,4,2,2,2,2. Find the area of the octagon. Bonus points: Find the radius of the circle.
Area = 42.627

Or in closed form...

Area = 20 + 16 * sqrt(2)
Radius = sqrt(10 + 4 * sqrt(2))

Is (14n+3)/(21n + 4) a fraction in simplest form for every n?
Yes. Argument...

gcd(14n+3, 21n + 4)
= gcd(14n+3, 7n + 1)
= gcd(0n+1, 7n + 1)
= 1

#### Bomb#20

##### Contributor
4. Two unit circles pass through each other's center. Find the area of their intersection. Bonus points: Same question for 3 intersecting circles.
Two: 1.228
Three: 0.7048

Or in closed form...

Inscribe a hexagon in a unit circle. It is composed of six unit-sided equilateral triangles. Let A be the area of one of them; let B be the area of one of the six slivers outside the hexagon but inside the circle. Then

A = sqrt(3) / 4
B = pi / 6 - A

By inspection, the intersection of two unit circles is 2A + 4B; the intersection of three unit circles is A + 3B.

#### Bomb#20

##### Contributor
I am an empiricist and I care not for damn proofs!

Well then, I guess you could empirically determine the sum of the squares of the roots of the polynomial equation a5x5+a4x4+a3x3+a2x2+a1x+a0=0.
Why, can you do it analytically?

ETA: Hey, Newton's Identities! Cool! And I'd thought Galois's proof meant we were screwed for the general quintic equation.

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#### beero1000

##### Veteran Member
Well then, I guess you could empirically determine the sum of the squares of the roots of the polynomial equation a5x5+a4x4+a3x3+a2x2+a1x+a0=0.
Why, can you do it analytically?

ETA: Hey, Newton's Identities! Cool! And I'd thought Galois's proof meant we were screwed for the general quintic equation.

Fun, right? (AKA )

#### beero1000

##### Veteran Member
1. If x,y,z are positive real numbers with xyz = 1, show that x2+y2+z2 ≤ x3+y3+z3
2. If n is a positive even number, is 23n-1 + 5(3n) divisible by 11?
3. What is $$\displaystyle \lim_{n\to\infty} \int_{1/(n+1)}^{1/n} \frac{\sin x}{x^3} dx$$?
4. Find the surface area and volume of a horn torus (a circle rotated about one of its tangents).

#### ryan

##### Veteran Member
An irregular octagon is inscribed in a circle. The edge lengths of the octagon are 4,4,4,4,2,2,2,2. Find the area of the octagon. Bonus points: Find the radius of the circle.
What did I do wrong?

4b + 4c = 2pi, where b is the arc of the 2 unit long line segment, and c is the other.

c = pi/2 - b

r = 2/sin(c)

r = 1/sin(b)

2sin(b) = sin(c)

2sin(b) = sin(pi/2 - b) = cos(b)

b = tan^(-1)(1/2)

r = 1/sin(b) = 2.236...

#### Bomb#20

##### Contributor
2. If n is a positive even number, is 23n-1 + 5(3n) divisible by 11?
Yes.

Change variables and write the formula mod 11. Let n = 2m+2, m a non-negative integer. So

23n-1 + 5(3n) = 26m+5 + 5(32m+2)
= 32(26)m + 5(9)(32)m
= -1(9m) + 1(9m) (mod 11)
= 0 (mod 11)

#### ryan

##### Veteran Member
2. If n is a positive even number, is 23n-1 + 5(3n) divisible by 11?
Yes.

Change variables and write the formula mod 11. Let n = 2m+2, m a non-negative integer. So

23n-1 + 5(3n) = 26m+5 + 5(32m+2)
= 32(26)m + 5(9)(32)m
= -1(9m) + 1(9m) (mod 11)
= 0 (mod 11)

I used induction.

n = 2m, where m is any natural number.

Assume, 26m - 1 + 5(32m) = 11q, where q is another natural number.

m = 1: 77 = 11q

m + 1: 26m + 5 + 5(32m + 2) = 26m - 1 + 5(32m + 2) + 11

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#### Bomb#20

##### Contributor
I used induction.

n = 2m, where m is any natural number.

Assume, 26m - 1 + 5(32m) = 11q, where q is another natural number.

m = 1: 77 = 11q

m + 1: 26m + 5 + 5(32m + 2) = 26m - 1 + 5(32m + 2) + 11

You must have dropped a term -- for m=1, that equation works out to 2453 = 448.

(Which isn't to say that induction couldn't work.)

What did I do wrong? ... sin(c) ...
Tried to use trig!

(Which isn't to say that trig couldn't work. But I got away with using the Pythagorean Theorem and a couple of pages of algebra.)

#### ryan

##### Veteran Member
You must have dropped a term -- for m=1, that equation works out to 2453 = 448.

(Which isn't to say that induction couldn't work.)

thanks

Let/assume 26m - 1 + 5(32m) = 11q.

m + 1: 26(26m - 1) + 5(32m + 2

= 26(26m - 1) + 32*5(32m)

= 26(26m - 1) + 9(11q - 26m - 1)

= 26(26m - 1) + 9(11q) - 9(26m - 1)

= 26m - 1(26 - 9) + 9(11q)

= 26m - 1(55) + 9(11q)

= 11((26m - 1)(5) + 9q)

What did I do wrong? ... sin(c) ...
Tried to use trig!

(Which isn't to say that trig couldn't work. But I got away with using the Pythagorean Theorem and a couple of pages of algebra.)

But where did I go wrong? Every step seems correct.

#### Bomb#20

##### Contributor
1. If x,y,z are positive real numbers with xyz = 1, show that x2+y2+z2 ≤ x3+y3+z3
Assume without loss of generality that x <= y <= z. z = 1/xy. If all three were less than 1 their product would be less than 1; likewise, if all three were greater than 1 their product would be greater than 1. Therefore x <= 1 and z >= 1. There are two cases to consider, y <= 1 and y > 1. Here's a proof for y <= 1.

To show that x2+y2+z2 <= x3+y3+z3 is to show that f = x3+y3+z3 - (x2+y2+z2) >= 0.

Let r = 1 - x, s = 1 - y, t = z - 1. r, s, t >= 0.

x(1 + r/x) = x + r = x + 1 - x = 1. Therefore 1 + r/x = 1/x. Similarly, 1 + s/y = 1/y.

z = 1/xy = (1/x)(1/y) = (1 + r/x)(1 + s/y) = 1 + r/x + s/y + rs/xy

t = r/x + s/y + rs/xy

f = x3+y3+z3 - (x2+y2+z2)

f = (1-r)3+(1-s)3+(1+t)3 - ((1-r)2+(1-s)2+(1+t)2)

f = (3 + 3(t-r-s) + 3(t2+r2+s2) + t3-r3-s3)
- (3 + 2(t-r-s) + t2+r2+s2)

f = t-r-s + 2(t2+r2+s2) + t3-r3-s3

Let g = 2(t2+r2+s2)

f = r/x + s/y + rs/xy - r - s + g + (r/x + s/y + rs/xy)3-r3-s3

(r/x + s/y + rs/xy)3 = (r/x)3 + (s/y)3 + [a series of 25 more products of sets of three terms].

Let h = that series of 25 products I'm not going to painfully write out.

f = r/x + s/y + rs/xy - r - s + g + (r/x)3 + (s/y)3 + h - r3 - s3

f = (r/x - r) + (s/y - s) + rs/xy + g + ((r/x)3 - r3) + ((s/y)3 - s3) + h

f = r(1/x - 1) + s(1/y - 1) + rs/xy + g + r3((1/x)3 - 1) + s3((1/y)3 - 1) + h

f = r(1 + r/x - 1) + s(1 + s/y - 1) + rs/xy + g + r3((1 + r/x)3 - 1) + s3((1 + s/y)3 - 1) + h

f = r(r/x) + s(s/y) + rs/xy + g + r3((1 + r/x)3 - 1) + s3((1 + s/y)3 - 1) + h

By inspection, every individual term in the above sum of terms is non-negative. Therefore f >= 0. Q.E.D.

The proof for y > 1 is analogous and is left as an exercise for the masochist.

#### beero1000

##### Veteran Member
You must have dropped a term -- for m=1, that equation works out to 2453 = 448.

(Which isn't to say that induction couldn't work.)

Induction will work, as long as he is careful with his algebra. As written, there are errors.

What did I do wrong? ... sin(c) ...
Tried to use trig!

(Which isn't to say that trig couldn't work. But I got away with using the Pythagorean Theorem and a couple of pages of algebra.)

Trig will work, if you're careful, but I agree it's not the best approach.

The "sneaky" approach:

Note that the order of the sides does not matter, so take the octagon side sequence to be alternating 4's and 2's. Take a square of side s and symmetrically chop off the corners at 45 degrees. You are left with an octagon that has two groups of four equal edges, and it is clearly cyclic. We want the small sides to have length 2, so we cut $$\sqrt{2}$$ off each edge. Then we want $$s - 2 \sqrt{2} = 4$$, so $$s = 4 + 2\sqrt{2}$$. Therefore, the area is $$(4+2\sqrt{2})^2 - 4 = 20 + 16\sqrt{2}$$.

The circle's radius is then $$\sqrt{(2+\sqrt{2})^2 + 2^2} = \sqrt{10 + 4\sqrt{2}}$$

#### ryan

##### Veteran Member
Trig will work, if you're careful, but I agree it's not the best approach.

I am so frustrated because each step seems right. Please - I beg you - take me out of my misery!

#### beero1000

##### Veteran Member
Trig will work, if you're careful, but I agree it's not the best approach.

I am so frustrated because each step seems right. Please - I beg you - take me out of my misery!

r = 2/sin(c/2) not r = 2/sin(c)

#### ryan

##### Veteran Member
I am so frustrated because each step seems right. Please - I beg you - take me out of my misery!

r = 2/sin(c/2) not r = 2/sin(c)

Oh for the love of ...

Thanks!!!!!!

#### beero1000

##### Veteran Member
1. If x,y,z are positive real numbers with xyz = 1, show that x2+y2+z2 ≤ x3+y3+z3
Assume without loss of generality that x <= y <= z. z = 1/xy. If all three were less than 1 their product would be less than 1; likewise, if all three were greater than 1 their product would be greater than 1. Therefore x <= 1 and z >= 1. There are two cases to consider, y <= 1 and y > 1. Here's a proof for y <= 1.

To show that x2+y2+z2 <= x3+y3+z3 is to show that f = x3+y3+z3 - (x2+y2+z2) >= 0.

Let r = 1 - x, s = 1 - y, t = z - 1. r, s, t >= 0.

x(1 + r/x) = x + r = x + 1 - x = 1. Therefore 1 + r/x = 1/x. Similarly, 1 + s/y = 1/y.

z = 1/xy = (1/x)(1/y) = (1 + r/x)(1 + s/y) = 1 + r/x + s/y + rs/xy

t = r/x + s/y + rs/xy

f = x3+y3+z3 - (x2+y2+z2)

f = (1-r)3+(1-s)3+(1+t)3 - ((1-r)2+(1-s)2+(1+t)2)

f = (3 + 3(t-r-s) + 3(t2+r2+s2) + t3-r3-s3)
- (3 + 2(t-r-s) + t2+r2+s2)

f = t-r-s + 2(t2+r2+s2) + t3-r3-s3

Let g = 2(t2+r2+s2)

f = r/x + s/y + rs/xy - r - s + g + (r/x + s/y + rs/xy)3-r3-s3

(r/x + s/y + rs/xy)3 = (r/x)3 + (s/y)3 + [a series of 25 more products of sets of three terms].

Let h = that series of 25 products I'm not going to painfully write out.

f = r/x + s/y + rs/xy - r - s + g + (r/x)3 + (s/y)3 + h - r3 - s3

f = (r/x - r) + (s/y - s) + rs/xy + g + ((r/x)3 - r3) + ((s/y)3 - s3) + h

f = r(1/x - 1) + s(1/y - 1) + rs/xy + g + r3((1/x)3 - 1) + s3((1/y)3 - 1) + h

f = r(1 + r/x - 1) + s(1 + s/y - 1) + rs/xy + g + r3((1 + r/x)3 - 1) + s3((1 + s/y)3 - 1) + h

f = r(r/x) + s(s/y) + rs/xy + g + r3((1 + r/x)3 - 1) + s3((1 + s/y)3 - 1) + h

By inspection, every individual term in the above sum of terms is non-negative. Therefore f >= 0. Q.E.D.

The proof for y > 1 is analogous and is left as an exercise for the masochist.

Ouch!

#### Bomb#20

##### Contributor
The "sneaky" approach:

Note that the order of the sides does not matter, so take the octagon side sequence to be alternating 4's and 2's. Take a square of side s and symmetrically chop off the corners at 45 degrees. You are left with an octagon that has two groups of four equal edges, and it is clearly cyclic. We want the small sides to have length 2, so we cut $$\sqrt{2}$$ off each edge. Then we want $$s - 2 \sqrt{2} = 4$$, so $$s = 4 + 2\sqrt{2}$$. Therefore, the area is $$(4+2\sqrt{2})^2 - 4 = 20 + 16\sqrt{2}$$.

The circle's radius is then $$\sqrt{(2+\sqrt{2})^2 + 2^2} = \sqrt{10 + 4\sqrt{2}}$$

That's a lot smarter than what I did.

...
The proof for y > 1 is analogous and is left as an exercise for the masochist.

Ouch!
Ouch indeed. So is it safe to assume there's a "sneaky" approach to the xyz=1 problem too?

#### Kharakov

##### Quantum Hot Dog
1. If x,y,z are positive real numbers with xyz = 1, show that x2+y2+z2 ≤ x3+y3+z3
Assume without loss of generality that x <= y <= z. z = 1/xy. If all three were less than 1 their product would be less than 1; likewise, if all three were greater than 1 their product would be greater than 1. Therefore x <= 1 and z >= 1. There are two cases to consider, y <= 1 and y > 1. Here's a proof for y <= 1.

The proof for y > 1 is analogous and is left as an exercise for the masochist.
That helped.

I do it the opposite way x>=y>=z, which may be a character flaw... anyway.

$$x^2 (x-1) + y^2 (y-1) + z^2 (z-1) \geq 0$$

if x and y are greater than 1, the only negative is the z term, and it will be smaller than 1. whoops...

If x>1 and y = 1, we only have the x and z terms to worry about. The x term will be larger because squaring increases the value of the x term, and decreases the value of the z term.

if only x is greater than 1, the y and z portions will both be negative. y=1/xz z= 1/xy
Working on it.

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#### beero1000

##### Veteran Member
That's a lot smarter than what I did.

...
The proof for y > 1 is analogous and is left as an exercise for the masochist.

Ouch!
Ouch indeed. So is it safe to assume there's a "sneaky" approach to the xyz=1 problem too?

Safe enough.

Based on two simple and very useful inequalities:
1. : $$\sqrt[3]{xyz} \leq (x+y+z)/3$$
2. : A sum of products is maximized when the factors are matched in order.

By the AM-GM inequality:
$$x^2 + y^2 + z^2 = (x^2 + y^2 + z^2)(\sqrt[3]{xyz}) \leq (x^2 + y^2 + z^2)(x+y+z)/3$$

Multiplying through and using the rearrangement inequality on the factors $$x^2,y^2,z^2$$ and $$x,y,z$$:
$$(x^3 + y^3 + z^3) + (x^2y + y^2z + z^2x) + (x^2z + y^2x + z^2y) \leq 3(x^3 + y^3 + z^3)$$

Put them together to get $$x^2 + y^2 + z^2 \leq x^3 + y^3 + z^3$$.

In general, this technique shows that if $$x_1x_2 \dots x_k = 1$$ then $$x_1^n + x_2^n + \dots + x_k^n \leq x_1^m + x_2^m + \dots + x_k^m$$ for $$m \geq n > 0$$.

#### Artemus

##### Veteran Member
3. What is $$\displaystyle \lim_{n\to\infty} \int_{1/(n+1)}^{1/n} \frac{\sin x}{x^3} dx$$?
Taking the Taylor expansion of sin(x) around x = 0,

$$\displaystyle \lim_{n\to\infty} \int_{1/(n+1)}^{1/n} \frac{\sin x}{x^3} dx = \lim_{n\to\infty} \int_{1/(n+1)}^{1/n} \sum_{m=0}^\infty \frac{(-1)^m}{(2m+1)!} \frac{x^{2m+1}}{x^3} dx$$
$$= \lim_{n\to\infty}\sum_{m=0}^\infty \frac{(-1)^m}{(2m+1)!} \int_{1/(n+1)}^{1/n} x^{2m - 2}\, dx$$

Since the integration limits both go to zero, the only contribution can be from the terms that approach a singularity as x goes to zero. Therefore, only the m = 0 term contributes, giving
$$\displaystyle = \lim_{n\to\infty} \int_{1/(n+1)}^{1/n} x^{- 2}\, dx =\lim_{n\to\infty} (-x)^{-1} |_{x = 1/(n+1)}^{1/n} = \lim_{n\to\infty}-[n - (n+1)] = 1.$$

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#### lpetrich

##### Contributor
I came rather later to this party, so I've ended up duplicating others' solutions.

Suppose the roots of the polynomial x5 + 2x4 + 3x3 + 4x2 + 5x + 6 are a, b, c, d, and e, respectively. Find a2+b2+c2+d2+e2. Bonus points: Find the harmonic mean of the roots (i.e. 5 times the reciprocal of the sum of their reciprocals).
One can avoid solving the equation by using .

Notation:
en = sub of n x's all distinct -- these appear in the polynomial's coefficients
pn = sum of the nth power of each x

We want p2:
e1 = - 2
e2 = 3
p1 = e1
p2 = e1*p1 - 2*e2
Thus,
p1 = -2
p2 = -2

For the harmonic mean, we find the polynomial for 1/x to get the reciprocals of the original one's roots:

(1/x)5 + (5/6)(1/x)4 + (2/3)(1/x)3 + (1/2)(1/x)2 + (1/3)(1/x) + (1/6)
e1 = -5/6
Mean = 1/5 of this, because it has 5 roots, or -1/6
Reciprocal of that mean = harmonic mean of original roots = -6

An irregular octagon is inscribed in a circle. The edge lengths of the octagon are 4,4,4,4,2,2,2,2. Find the area of the octagon. Bonus points: Find the radius of the circle.
The solution turns out to be independent of their order in the circle. For side length s and radius r, the angle subtended by it at the origin is 2*arcsin(s/(2r)). So, arcsin(1/r) + arcsin(2/r) = pi/4.

The area of each side triangle: (s/2)*sqrt(r^2 - (s/2)^2)

Area = 4 * (2*sqrt(2) + 1) + 4 * (2 * (2 + sqrt(2))) = 20 + 16*sqrt(2) = 4 * (5 + 4*sqrt(2))

Is (14n+3)/(21n + 4) a fraction in simplest form for every n?
Presumably integer n.

Let's use Euclid's algorithm for the GCD:
21n + 4
14n + 3
7n + 1
1

The numerator and the denominator are thus always relatively prime, making the fraction always in lowest terms.

Two unit circles pass through each other's center. Find the area of their intersection. Bonus points: Same question for 3 intersecting circles.
If their centroid is at {0,0} and their centers are at {1/2,0}, and {-1/2,0}, then their intersection points are at {0,sqrt(3)/2} and {0,-sqrt(3)/2}

The circle slice from center to each intersection point has angle 2*60d = 120d = 2pi/3. That gives an area of (1/2)*this or pi/3 The triangle between those points has area sqrt(3)/4. Their difference: pi/3 - sqrt(3)/4. The intersection region is twice this, with area
2*pi/3 - sqrt(3)/2 ~ 1.22837

For the 3-circle case, we take one of the circles, with center {-1/2, 0}. The part of it shared by the others is a circle slice with angle pi/3 - triangle {-1/2,0} - {1/2,0} - {0,sqrt(3)/2} + triangle {1/2,0} - {0,sqrt[3}/2} - {0,sqrt{3}/6}

That's pi/6 - sqrt(3)/4 + sqrt(3)/12 = pi/6 - sqrt(3)/6

For all three together, we get
pi/2 - sqrt(3)/2 ~ 0.7046

Both results were checked numerically.

#### lpetrich

##### Contributor
f x,y,z are positive real numbers with xyz = 1, show that x2+y2+z2 ≤ x3+y3+z3

One can solve this by finding the minimum of
$$(x^3+y^3+z^3) - (x^2+y^2+z^2) - \lambda xyz$$

putting in the constraint with a Lagrange multiplier. Taking derivatives gives
$$x^2(3x-2) = y^2(3y-2) = z^2(3z-2) = \lambda$$

Crunching with Mathematica yields only one real solution: x = y = z = 1.

Since the derivatives are nonzero elsewhere, the inequality is always in one direction elsewhere. For x = 2, y = 1/2, z = 1, the lhs is 21/4, and the rhs is 73/8, and rhs > lhs. Thus, rhs > lhs everywhere.

If n is a positive even number, is 23n-1 + 5(3n) divisible by 11?

Consider the function f = 26n-1 + 5*32n

It satisfies recurrence relation f(n+2) -73*f(n+1) + 576*f = 0 -- it's a 3-term one because the function has 2 exponentials in it.

So if any two consecutive ones are evenly divisible by 11, all the rest are. f(1) = 77 = 7*11, f(2) = 2453 = 223*11, and thus all the rest are.

What is $$\displaystyle \lim_{n\to\infty} \int_{1/(n+1)}^{1/n} \frac{\sin x}{x^3} dx$$?

Since x is small, it becomes $$\displaystyle \lim_{n\to\infty} \int_{1/(n+1)}^{1/n} \frac{1}{x^2} + O(1) \, dx = \lim_{n\to\infty} (n+1) - n + O(1/n^2) = 1$$

Find the surface area and volume of a horn torus (a circle rotated about one of its tangents).

$$\displaystyle A = 2 \pi \int \sqrt{ 1 + \left( \frac{dr}{dz} \right)^2} r \, dz$$
$$\displaystyle V = \pi \int (r_{outer}(z)^2 - r_{inner}(z)^2) \, dz$$

$$A = 4\pi^2 a b ,\ V = 2\pi a b^2$$

Here, a = b.

#### beero1000

##### Veteran Member
Bored today and thinking about spirals. Find the arc-length formulas for Archimedes' spiral (r = c θ):
1. On the Euclidean plane
2. On the Elliptic plane
3. On the Hyperbolic plane
Solver's choice on details for the metrics/models and FYI, calculation-wise, these are not as nice as my usual questions...

#### Kharakov

##### Quantum Hot Dog
I solved the |c|=0 case. I'll leave the |c| > 0 case for the rest of you.

1. $$c (\theta \sin(\theta)+\sin(\theta)-\theta \cos(\theta)+\cos(\theta))$$

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#### lpetrich

##### Contributor
First, LaTeX/Mathematics - Wikibooks, open books for an open world goes into detail about LaTeX.

My solution:

First, the spaces that beero1000 listed all have this metric:
$$ds^2 = dr^2 + S(r)^2 d\theta^2$$
where s is the distance, r and θ are polar coordinates, and
$$S(r) = \left\{ \begin{array}{l l} R \sin(r/R) & \text{elliptic} \\ r & \text{flat} \\ R \sinh(r/R) & \text{hyperbolic} \end{array} \right.$$

he integral for distance is
$$\displaystyle s = \int \sqrt{c^2 + S(c\theta)^2} \,d\theta$$

In the flat case,
$$\displaystyle s = c \int \sqrt{1 + \theta^2} \,d\theta = \frac{c}{2} \left( \theta \sqrt{1 + \theta^2} + \text{arcsinh} \theta \right)$$

The non-flat cases require Jacobi elliptic integrals, and those are rather complicated.

#### lpetrich

##### Contributor
The elliptic version

It uses Jacobi elliptic functions and integrals.
$$\displaystyle s = \int \sqrt{ c^2 + R^2 \sin^2 ((c/R) \theta) } \, d\theta = c E(m,\phi) ,\ m = - (R/c)^2 ,\ \phi = (c/R)\theta$$
E is the second kind of elliptic integral.

One can get the hyperbolic case by analytic continuation.

#### beero1000

##### Veteran Member
Classes just ended and I've suddenly got a lot more time...
1. Find the largest area rectangle inside a semicircle.
2. Find the largest area triangle inside a semicircle.
3. Find the largest area semicircle inside a unit square.
4. Find the 100th digit after the decimal of $$(1 + \sqrt{2})^{2000}$$
5. The largest known prime is p = 257,885,161 − 1. Find the last (least significant) digit of p. Find the first (most significant) digit of p.

... and one of my favorites:

1. Given 10 points in the plane, can they always be covered by (at most) 10 disjoint unit disks?

#### Bomb#20

##### Contributor
5. The largest known prime is p = 257,885,161 − 1. Find the last (least significant) digit of p. Find the first (most significant) digit of p.
Too easy. The first digit is 1. The last digit is f.

#### George S

##### Veteran Member
• The largest known prime is p = 257,885,161 − 1. Find the last (least significant) digit of p. Find the first (most significant) digit of p.
Too easy. The first digit is 1. The last digit is f.

Easier: Both the first and last digit are 1 (in base 2). 2x-1 = 1, 11, 111, 1111, 11111...1111

#### Kharakov

##### Quantum Hot Dog
Too easy. The first digit is 1. The last digit is f.

Easier: Both the first and last digit are 1 (in base 2). 2x-1 = 1, 11, 111, 1111, 11111...1111

Keeping with base 2....

$$\sqrt{ 10 - \sqrt{10+\sqrt{10+\sqrt{10+\sqrt{10+\sqrt{10 + \dots }}}}}}$$

Produces a specific pattern of base 2 digits after a whole bunch of zeros.
The number of times one must shift the pattern to the left, past the decimal point a bit, to match a familiar number, matches the number of 0s in the equation.
If the symbol for zero was any rounder, my reasoning would be even more circular.

I can't Pell if I should write out a formula for a specific number to multiply by the square root of 2, for the 2tooth question. 2 punny? I assume a simple entertaining trick, because it's you. Ohh.. hahhaa.. nm. Awesome.

I believe the answer to the question "Is there no trick?" is no, in German.

1 and 2 must be r^2. 3. I cut the pizza 4 times, and took one slice.

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#### lpetrich

##### Contributor
Classes just ended and I've suddenly got a lot more time...
Thanx.

Find the largest area rectangle inside a semicircle.

The rectangle must fit against the straight side, and it must extend to the circular side. Will use unit radius for convenience. For w along the straight side and h perpendicular to it, we must minimize area A
$$A = 2wh ,\ w^2 + h^2 = 1$$
Easy to do with Lagrange multipliers:
$$2h - 2\lambda w = 0 ,\ 2w - 2\lambda h = 0$$
Solution: w = h = 1/sqrt(2), λ = 1.

Find the largest area triangle inside a semicircle.

Two of the points are at the line-circle intersections, while the third one is on the circle part, at along-line offset from center w and perpendicular offset h. The triangle's area is (1/2)*2*h = h. So the maximum-area triangle is a right triangle with the right-angle vertex on a line perpendicular from the center.

Find the largest area semicircle inside a unit square.

Let the square's vertices be A11 - A12 - A22 - A21 and back to A11. The straight side intersects A11 - A12 at B1 and A11 - A21 at B2, and the circular side touches A12 - A22 at C1 and A21 - A22 at C2. Let the circle's radius be r and its center be O.

Let angle B1 - O - C1 be a. Then B2 - O - C2 will be 90d - a. Projecting from C2 to O to A11 - A12 gives total length r*(1 + sin(a)), and doing likewise from C1 to O to A11 - A21 gives total length r*(1 + cos(a)). This means sin(a) = cos(a) or a = 45d. That also yields radius r = 2 - sqrt(2).

Find the 100th digit after the decimal of $$(1 + \sqrt{2})^{2000}$$

This number is approximatly 10765.551 and its reciprocal 10-765.551. However, this number can be expressed in form a + b, where a is an integer and b an integer times sqrt(2). Its reciprocal is likewise a - b. Thus, b = a - 10-765.551 and a + b = 2a - 10-765.551. Since a is an integer, the number must have about 765 decimal digits that are 9. Thus, the 100th digit is 9.

The largest known prime is p = 257,885,161− 1. Find the last (least significant) digit of p. Find the first (most significant) digit of p.

When one takes powers of 2, the digits cycle as follows:
4p: 6
4p+1: 2
4p+2: 4
4p+3: 8

Since the exponent has form 4p+1, the least significant digit thus equals 2 - 1 = 1.

I found the most significant digit by brute force using Mathematica. It is 5.

Given 10 points in the plane, can they always be covered by (at most) 10 disjoint unit disks?

[1101.3468] Packing-constrained point coverings considers what number of points have some configuration that cannot be covered by that number of unit disks. The number is >= 11 and <= 55. So it's always possible for 10 points.

#### beero1000

##### Veteran Member
The largest known prime is p = 257,885,161− 1. Find the last (least significant) digit of p. Find the first (most significant) digit of p.

When one takes powers of 2, the digits cycle as follows:
4p: 6
4p+1: 2
4p+2: 4
4p+3: 8

Since the exponent has form 4p+1, the least significant digit thus equals 2 - 1 = 1.

I found the most significant digit by brute force using Mathematica. It is 5.

For the most significant digit:

Look at the fractional part of 57885161 log102...

Given 10 points in the plane, can they always be covered by (at most) 10 disjoint unit disks?

[1101.3468] Packing-constrained point coverings considers what number of points have some configuration that cannot be covered by that number of unit disks. The number is >= 11 and <= 55. So it's always possible for 10 points.

The current best known bounds are 13 and 45, respectively . However, there is a particularly nice argument for the n ≤ 10 case.

#### beero1000

##### Veteran Member
Covering 10 points with disjoint circles.

Take a regular packing of circles, and center it randomly in the plane. Note that the proportion of the plane that is covered by the circles is $$\pi/2\sqrt{3} \approx 0.9069$$, so the expected number of points covered by the circles is about 9.069. Since 9.069 > 9, there must exist packing placements that cover all 10 points.

Isn't the probabilistic method wonderful?

#### beero1000

##### Veteran Member
I recently found http://www.brilliant.org. It is a training site for high school math competitions. You work up in levels to get to harder problems.

Fun!

#### Kharakov

##### Quantum Hot Dog
Are there any proofs that certain mathematical formulas cannot arise in nature (without a conscious agent) due to the nature of the formulas? Is anyone working on such a proof?

#### Kharakov

##### Quantum Hot Dog
Not formulas, patterns.

#### beero1000

##### Veteran Member
I'd say that the fit the bill.

#### Kharakov

##### Quantum Hot Dog
I was thinking more along the lines of geometric transforms that couldn't occur in nature. I don't think there is a mathematical proof that a reflection across an axis cannot occur in nature, so perhaps asking for a proof is a bit over the top, although perhaps there is a proof that according to the current understanding of natural law (that I am aware of), certain transforms do not occur in nature.

I don't think relativity will allow something located at (-1,-1,-1) to be reflected to (1,-1,-1). While we can imagine this reflection happening, or simulate it on a computer, I don't think we will find this type of transform occurring in nature outside of a mind or a simulation.

#### Bomb#20

##### Contributor
I'd say that the fit the bill.
You mean because they can't arise in nature, period? I expect Kharakov had in mind a pattern that can arise with a conscious agent. It seems to me what would fit the bill is a core dump of a running fully detailed simulation of a human brain.

#### Bomb#20

##### Contributor
I was thinking more along the lines of geometric transforms that couldn't occur in nature. I don't think there is a mathematical proof that a reflection across an axis cannot occur in nature, so perhaps asking for a proof is a bit over the top, although perhaps there is a proof that according to the current understanding of natural law (that I am aware of), certain transforms do not occur in nature.

I don't think relativity will allow something located at (-1,-1,-1) to be reflected to (1,-1,-1). While we can imagine this reflection happening, or simulate it on a computer, I don't think we will find this type of transform occurring in nature outside of a mind or a simulation.
Do not occur or could not occur? Big difference. For an object to spontaneously be reflected across the Y-Z plane is no more a violation of known natural law than for all the molecules in the hostess' undergarments to leap one foot simultaneously to the left. In quantum tunneling, all things are possible. The best you'll get is a proof that the expected time you'd probably have to wait before witnessing such an event exceeds the lifetime of the universe.

#### jonatha

##### Veteran Member
A naturally occurring prime number generator seems very farfetched....

#### Kharakov

##### Quantum Hot Dog
I'd say that the fit the bill.
You mean because they can't arise in nature, period? I expect Kharakov had in mind a pattern that can arise with a conscious agent. It seems to me what would fit the bill is a core dump of a running fully detailed simulation of a human brain.
That's one of the trivial cases.

I'm looking for a basic mathematical operation that indicates a consciousness engaged in the operation because natural law prevents it from happening.

#### Kharakov

##### Quantum Hot Dog
I was thinking more along the lines of geometric transforms that couldn't occur in nature. I don't think there is a mathematical proof that a reflection across an axis cannot occur in nature, so perhaps asking for a proof is a bit over the top, although perhaps there is a proof that according to the current understanding of natural law (that I am aware of), certain transforms do not occur in nature.

I don't think relativity will allow something located at (-1,-1,-1) to be reflected to (1,-1,-1). While we can imagine this reflection happening, or simulate it on a computer, I don't think we will find this type of transform occurring in nature outside of a mind or a simulation.
Do not occur or could not occur?
Think classical scale.

#### Kharakov

##### Quantum Hot Dog
A naturally occurring prime number generator seems very farfetched....

Ahh, but primes are naturally selected for in certain scenarios, so one would have to pick something other than primes to indicate consciousness did the selection:
https://en.wikipedia.org/wiki/Prime_number#Prime_numbers_in_nature

If someone encoded primes into certain DNA sequences, a metaphysical naturalist would claim that the primes just come up because of evolutionary adaptions that at one time favored primes being selected, using the obvious example of cicadas.