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Level 2

Let n 5 n \geq 5 be a fixed odd positive integer and extend the above pentagrams to n n grams.

For each positive integer m m , let P m P_{m} be the n n gram inscribed in the circle C m C_{m} and C m + 1 C_{m + 1} be the circle inscribed in the n n gram P m P_{m} .

Let A c ( m ) A_{c}(m) be the area of each circle C m C_{m} and A p ( m ) A_{p}(m) be the area of each n n gram P m P_{m} and let A c ( n ) = m = 1 A c ( m ) A_{c}(n) = \sum_{m = 1}^{\infty} A_{c}(m) and A c = lim n A c ( n ) A_{c} = \lim_{n \rightarrow \infty} A_{c}(n) and let A p ( n ) = m = 1 A p ( m ) A_{p}(n) = \sum_{m = 1}^{\infty} A_{p}(m) and A p = lim n A p ( n ) A_{p} = \lim_{n \rightarrow \infty} A_{p}(n) .

If A c A p A c ( 1 ) 2 = ( a b ) a \dfrac{A_{c} * A_{p}}{A_{c}(1)^2} = (\dfrac{a}{b})^{a} , where a a and b b are coprime positive integers, find a + b a + b .


The answer is 7.

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1 solution

Rocco Dalto
Feb 4, 2019

Let n 5 n \geq 5 be an odd positive integer.

Note: This is simpler to type then using 2 n + 3 2n + 3 for each positive integer n n .

Let O P = r 1 OP = r_{1} and O C = x P C = r 1 x OC = x \implies PC = r_{1} - x .

m A O P = π n m\angle{AOP} = \dfrac{\pi}{n} and by Inscribed Angle Theorem m E P F = 1 2 m E O F = 1 2 ( 2 π n ) = π n m A P O = π 2 n m\angle{EPF} = \dfrac{1}{2}m\angle{EOF} = \dfrac{1}{2}(\dfrac{2\pi}{n}) = \dfrac{\pi}{n} \implies m\angle{APO} = \dfrac{\pi}{2n} .

h r 1 x = tan ( π 2 n ) h = ( r 1 x ) tan ( π 2 n ) \dfrac{h}{r_{1} - x} = \tan(\dfrac{\pi}{2n}) \implies h = (r_{1} - x)\tan(\dfrac{\pi}{2n}) and h = x tan ( π n ) ( r 1 x ) tan ( π 2 n ) = x tan ( π n ) r 1 tan ( π 2 n ) = ( tan ( π n ) + tan ( π 2 n ) ) x h = x \tan(\dfrac{\pi}{n}) \implies (r_{1} - x)\tan(\dfrac{\pi}{2n}) = x\tan(\dfrac{\pi}{n}) \implies r_{1}\tan(\dfrac{\pi}{2n}) = (\tan(\dfrac{\pi}{n}) + \tan(\dfrac{\pi}{2n}))x x = tan ( π 2 n ) tan ( π n ) + tan ( π 2 n ) r 1 h = tan ( π n ) tan ( π 2 n ) tan ( π n ) + tan ( π 2 n ) r 1 \implies x = \dfrac{\tan(\dfrac{\pi}{2n})}{\tan(\dfrac{\pi}{n}) + \tan(\dfrac{\pi}{2n})} r_{1} \implies h = \dfrac{\tan(\dfrac{\pi}{n})\tan(\dfrac{\pi}{2n})}{\tan(\dfrac{\pi}{n}) + \tan(\dfrac{\pi}{2n})} r_{1}

tan ( π n ) = tan ( 2 π n ) = 2 tan ( π 2 n ) 1 tan 2 ( π 2 n ) \tan(\dfrac{\pi}{n}) = \tan(\dfrac{2\pi}{n}) = \dfrac{2\tan(\dfrac{\pi}{2n})}{1 - \tan^2(\dfrac{\pi}{2n})} h = 2 tan ( π 2 n ) 3 tan 2 ( π 2 n ) r 1 \implies h = \dfrac{2\tan(\dfrac{\pi}{2n})}{3 - \tan^2(\dfrac{\pi}{2n})} r_{1}

Let B n = tan ( π 2 n ) 3 tan 2 ( π 2 n ) h = 2 B n r 1 = r 2 sin ( π n ) r 2 = 2 B n sin ( π n ) r 1 B_{n} = \dfrac{\tan(\dfrac{\pi}{2n})}{3 - \tan^2(\dfrac{\pi}{2n})} \implies h = 2B_{n}r_{1} = r_{2}\sin(\dfrac{\pi}{n}) \implies r_{2} = \dfrac{2B_{n}}{\sin(\dfrac{\pi}{n})}r_{1}

In General:

r m = ( 2 B n sin ( π n ) ) m 1 r 1 A c ( m ) = π r m 2 = π ( 4 B n 2 sin 2 ( π n ) ) m 1 r 1 2 r_{m} = (\dfrac{2B_{n}}{\sin(\dfrac{\pi}{n})})^{m - 1}r_{1} \implies A_{c}(m) = \pi r_{m}^2 = \pi(\dfrac{4B_{n}^2}{\sin^2(\dfrac{\pi}{n})})^{m - 1}r_{1}^2 \implies A c ( n ) = m = 1 A c ( m ) = sin 2 ( π n ) sin 2 ( π n ) 4 B n 2 π r 1 2 A_{c}(n) = \sum_{m = 1}^{\infty} A_{c}(m) = \dfrac{\sin^2(\dfrac{\pi}{n})}{\sin^2(\dfrac{\pi}{n}) - 4B_{n}^2}\pi r_{1}^2 = sin 2 ( π n ) sin 2 ( π n ) 4 B n 2 A c ( 1 ) \dfrac{\sin^2(\dfrac{\pi}{n})}{\sin^2(\dfrac{\pi}{n}) - 4B_{n}^2} A_{c}(1)

Let u = tan ( y 2 ) u 2 = 1 cos ( y ) 1 + cos ( y ) = sin 2 ( y ) ( 1 + cos ( y ) ) 2 u = sin ( y ) 1 + cos ( y ) u = \tan(\dfrac{y}{2}) \implies u^2 = \dfrac{1 - \cos(y)}{1 + \cos(y)} = \dfrac{\sin^2(y)}{(1 + \cos(y))^2} \implies u = \dfrac{\sin(y)}{1 + \cos(y)} and u 2 = 1 cos ( y ) 1 + cos ( y ) cos ( y ) = 1 u 2 1 + u 2 sin ( y ) = 2 u 1 + u 2 u^2 = \dfrac{1 - \cos(y)}{1 + \cos(y)} \implies \cos(y) = \dfrac{1 - u^2}{1 + u^2} \implies \sin(y) = \dfrac{2u}{1 + u^2} and u = tan ( y 2 ) u = \tan(\dfrac{y}{2})

Let y = π n u n = tan ( π 2 n ) B n = u n 3 u n 2 y = \dfrac{\pi}{n} \implies u_{n} = \tan(\dfrac{\pi}{2n}) \implies B_{n} = \dfrac{u_{n}}{3 - u_{n}^2} and sin ( π n ) = 2 u n 1 + u n 2 A c ( n ) = ( 3 u n 2 ) 2 ( 3 u n 2 ) 2 ( 1 + u n 2 ) 2 A c ( 1 ) \sin(\dfrac{\pi}{n}) = \dfrac{2u_{n}}{1 + u_{n}^2} \implies A_{c}(n) = \dfrac{(3 - u_{n}^2)^2}{(3 - u_{n}^2)^2 - (1 + u_{n}^2)^2} A_{c}(1) and lim n u n = 0 A c = lim n A c ( n ) = 9 8 A c ( 1 ) \lim_{n \rightarrow \infty} u_{n} = 0 \implies A_{c} = \lim_{n \rightarrow \infty} A_{c}(n) = \dfrac{9}{8}A_{c}(1)

From above h 1 = 2 B n r 1 h 2 = 2 B n r 2 = 2 B n ( 2 B n sin ( π n ) ) r 1 h 3 = 2 B n ( 2 B n sin ( π n ) ) 2 r 1 h_{1} = 2B_{n}r_{1} \implies h_{2} = 2B_{n}r_{2} = 2B_{n}(\dfrac{2B_{n}}{\sin(\dfrac{\pi}{n})})r_{1} \implies h_{3} = 2B_{n}(\dfrac{2B_{n}}{\sin(\dfrac{\pi}{n})})^2 r_{1}

In General:

h m = 2 B n ( 2 B n sin ( π n ) ) m 1 r 1 h_{m} = 2B_{n}(\dfrac{2B_{n}}{\sin(\dfrac{\pi}{n})})^{m - 1}r_{1} and from above r m = ( 2 B n sin ( π n ) ) m 1 r 1 A p ( m ) = 2 n ( 1 2 ) r m h m = r_{m} = (\dfrac{2B_{n}}{\sin(\dfrac{\pi}{n})})^{m - 1}r_{1} \implies A_{p}(m) = 2n(\dfrac{1}{2})r_{m}h_{m} = n ( 2 B n ) ( 4 B n 2 sin 2 ( π n ) ) m 1 r 1 2 n(2B_{n})(\dfrac{4B_{n}^2}{\sin^2(\dfrac{\pi}{n})})^{m - 1}r_{1}^2

A p ( n ) = m = 1 A p ( m ) = 2 n B n ( sin 2 ( π n ) sin 2 ( π n ) 4 B n 2 ) r 1 2 \implies A_{p}(n) = \sum_{m = 1}^{\infty} A_{p}(m) = 2nB_{n}(\dfrac{\sin^2(\dfrac{\pi}{n})}{\sin^2(\dfrac{\pi}{n}) - 4B_{n}^2})r_{1}^2 .

Let t n = 2 n B n = 2 n tan ( π 2 n ) 3 tan 2 ( π 2 n ) t_{n} = 2nB_{n} = 2n\dfrac{\tan(\dfrac{\pi}{2n})}{3 - \tan^2(\dfrac{\pi}{2n})} and S n = sin 2 ( π n ) sin 2 ( π n ) 4 B n 2 r 1 2 S_{n} = \dfrac{\sin^2(\dfrac{\pi}{n})}{\sin^2(\dfrac{\pi}{n}) - 4B_{n}^2} r_{1}^2

From above it was shown that lim n S n = 9 8 \lim_{n_\rightarrow \infty} S_{n} = \dfrac{9}{8}

For t n t_{n} :

Using the inequality cos ( x ) < sin ( x ) x < 1 \cos(x) < \dfrac{\sin(x)}{x} < 1 we have:

π cos ( π 2 n ) < 2 n sin ( π 2 n ) < π π < 2 n tan ( π 2 n ) < π sec ( π 2 n ) \pi\cos(\dfrac{\pi}{2n}) < 2n\sin(\dfrac{\pi}{2n}) < \pi \implies \pi < 2n\tan(\dfrac{\pi}{2n}) < \pi\sec(\dfrac{\pi}{2n}) and π lim n sec ( π 2 n ) = π lim n t n = π 3 \pi\lim_{n_\rightarrow \infty} \sec(\dfrac{\pi}{2n}) = \pi \implies \lim_{n_\rightarrow \infty} t_{n} = \dfrac{\pi}{3}

lim n A p ( n ) = lim n t n S n = π 3 9 8 = 3 8 π r 1 2 = 3 8 A c ( 1 ) \implies \lim_{n_\rightarrow \infty} A_{p}(n) = \lim_{n_\rightarrow \infty} t_{n} * S_{n} = \dfrac{\pi}{3} * \dfrac{9}{8} = \dfrac{3}{8}\pi r_{1}^2 = \dfrac{3}{8}A_{c}(1)

A c A p A c ( 1 ) 2 = 27 64 = ( 3 4 ) 3 = ( a b ) a a + b = 7 \implies \dfrac{A_{c} * A_{p}}{A_{c}(1)^2} = \dfrac{27}{64} = (\dfrac{3}{4})^3 = (\dfrac{a}{b})^{a} \implies a + b = \boxed{7}

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