Let n ≥ 5 be a fixed odd positive integer and extend the above pentagrams to n grams.
For each positive integer m , let P m be the n gram inscribed in the circle C m and C m + 1 be the circle inscribed in the n gram P m .
Let A c ( m ) be the area of each circle C m and A p ( m ) be the area of each n gram P m and let A c ( n ) = ∑ m = 1 ∞ A c ( m ) and A c = lim n → ∞ A c ( n ) and let A p ( n ) = ∑ m = 1 ∞ A p ( m ) and A p = lim n → ∞ A p ( n ) .
If A c ( 1 ) 2 A c ∗ A p = ( b a ) a , where a and b are coprime positive integers, find a + b .
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Let n ≥ 5 be an odd positive integer.
Note: This is simpler to type then using 2 n + 3 for each positive integer n .
Let O P = r 1 and O C = x ⟹ P C = r 1 − x .
m ∠ A O P = n π and by Inscribed Angle Theorem m ∠ E P F = 2 1 m ∠ E O F = 2 1 ( n 2 π ) = n π ⟹ m ∠ A P O = 2 n π .
r 1 − x h = tan ( 2 n π ) ⟹ h = ( r 1 − x ) tan ( 2 n π ) and h = x tan ( n π ) ⟹ ( r 1 − x ) tan ( 2 n π ) = x tan ( n π ) ⟹ r 1 tan ( 2 n π ) = ( tan ( n π ) + tan ( 2 n π ) ) x ⟹ x = tan ( n π ) + tan ( 2 n π ) tan ( 2 n π ) r 1 ⟹ h = tan ( n π ) + tan ( 2 n π ) tan ( n π ) tan ( 2 n π ) r 1
tan ( n π ) = tan ( n 2 π ) = 1 − tan 2 ( 2 n π ) 2 tan ( 2 n π ) ⟹ h = 3 − tan 2 ( 2 n π ) 2 tan ( 2 n π ) r 1
Let B n = 3 − tan 2 ( 2 n π ) tan ( 2 n π ) ⟹ h = 2 B n r 1 = r 2 sin ( n π ) ⟹ r 2 = sin ( n π ) 2 B n r 1
In General:
r m = ( sin ( n π ) 2 B n ) m − 1 r 1 ⟹ A c ( m ) = π r m 2 = π ( sin 2 ( n π ) 4 B n 2 ) m − 1 r 1 2 ⟹ A c ( n ) = ∑ m = 1 ∞ A c ( m ) = sin 2 ( n π ) − 4 B n 2 sin 2 ( n π ) π r 1 2 = sin 2 ( n π ) − 4 B n 2 sin 2 ( n π ) A c ( 1 )
Let u = tan ( 2 y ) ⟹ u 2 = 1 + cos ( y ) 1 − cos ( y ) = ( 1 + cos ( y ) ) 2 sin 2 ( y ) ⟹ u = 1 + cos ( y ) sin ( y ) and u 2 = 1 + cos ( y ) 1 − cos ( y ) ⟹ cos ( y ) = 1 + u 2 1 − u 2 ⟹ sin ( y ) = 1 + u 2 2 u and u = tan ( 2 y )
Let y = n π ⟹ u n = tan ( 2 n π ) ⟹ B n = 3 − u n 2 u n and sin ( n π ) = 1 + u n 2 2 u n ⟹ A c ( n ) = ( 3 − u n 2 ) 2 − ( 1 + u n 2 ) 2 ( 3 − u n 2 ) 2 A c ( 1 ) and lim n → ∞ u n = 0 ⟹ A c = lim n → ∞ A c ( n ) = 8 9 A c ( 1 )
From above h 1 = 2 B n r 1 ⟹ h 2 = 2 B n r 2 = 2 B n ( sin ( n π ) 2 B n ) r 1 ⟹ h 3 = 2 B n ( sin ( n π ) 2 B n ) 2 r 1
In General:
h m = 2 B n ( sin ( n π ) 2 B n ) m − 1 r 1 and from above r m = ( sin ( n π ) 2 B n ) m − 1 r 1 ⟹ A p ( m ) = 2 n ( 2 1 ) r m h m = n ( 2 B n ) ( sin 2 ( n π ) 4 B n 2 ) m − 1 r 1 2
⟹ A p ( n ) = ∑ m = 1 ∞ A p ( m ) = 2 n B n ( sin 2 ( n π ) − 4 B n 2 sin 2 ( n π ) ) r 1 2 .
Let t n = 2 n B n = 2 n 3 − tan 2 ( 2 n π ) tan ( 2 n π ) and S n = sin 2 ( n π ) − 4 B n 2 sin 2 ( n π ) r 1 2
From above it was shown that lim n → ∞ S n = 8 9
For t n :
Using the inequality cos ( x ) < x sin ( x ) < 1 we have:
π cos ( 2 n π ) < 2 n sin ( 2 n π ) < π ⟹ π < 2 n tan ( 2 n π ) < π sec ( 2 n π ) and π lim n → ∞ sec ( 2 n π ) = π ⟹ lim n → ∞ t n = 3 π
⟹ lim n → ∞ A p ( n ) = lim n → ∞ t n ∗ S n = 3 π ∗ 8 9 = 8 3 π r 1 2 = 8 3 A c ( 1 )
⟹ A c ( 1 ) 2 A c ∗ A p = 6 4 2 7 = ( 4 3 ) 3 = ( b a ) a ⟹ a + b = 7
.