An Odd-Only Generalization

Geometry Level 2

Assume that A 1 A n + 1 , A 2 A n + 2 , A 3 A n + 3 , A_1A_{n+1}, A_2A_{n+2}, A_3A_{n+3}, \ldots \ldots \ldots and A n A 2 n A_nA_{2n} are n 3 n\geq 3 concurrent lines, where n n is an odd positive integer. The point of concurrence is C C .

A counter-clockwise rotating ray with its endpoint fixed at C C and initial position along C A 1 CA_1 passes, in its first full rotation, A i A_i before A j A_j whenever i < j i < j .

Draw the segments A 1 A 2 , A 3 A 4 , A 5 A 6 A_1A_2, A_3A_4, A_5A_6 \ldots \ldots \ldots and A 2 n 1 A 2 n A_{2n-1}A_{2n} .

Now we define the following function F F .

F ( n ) = ( C A 1 A 2 + C A 2 A 1 ) + ( C A 3 A 4 + C A 4 A 3 ) + ( C A 5 A 6 + C A 6 A 5 ) + + ( C A 2 n 1 A 2 n + C A 2 n A 2 n 1 ) F(n)= (\angle CA_1A_2 + \angle CA_2A_1) + (\angle CA_3A_4 + \angle CA_4A_3) + (\angle CA_5A_6 + \angle CA_6A_5) + \ldots \ldots \ldots + (\angle CA_{2n-1}A_{2n} + \angle CA_{2n}A_{2n-1}) .

The following figure illustrates the case for n = 3 n=3 with the defining angles for F ( 3 ) F(3) indicated.

Find F ( 2017 ) F(2017) , in degrees.

Inspiration


The answer is 362880.

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1 solution

First notice, C A i A j \angle CA_iA_j contributes to F F if and only if i j = ± 1 i-j = \pm 1 and the minimum of i i and j j is an odd integer.

Now, we have, in degree measure, A 1 C A n + 1 = 180 \angle A_1CA_{n+1} = 180

[As A 1 , C , A n + 1 A_1, C, A_{n+1} are collinear]

A 1 C A 2 + A 2 C A 3 + A 3 C A 4 + A 4 C A 5 + + A n C A n + 1 = 180 \implies \angle A_1CA_2 + \angle A_2CA_3+ \angle A_3CA_4+ \angle A_{4}CA_{5}+ \ldots \ldots \ldots +\angle A_nCA_{n+1} = 180

A 1 C A 2 + A 2 + n C A 3 + n + A 3 C A 4 + A 4 + n C A 5 + n + + A n C A n + 1 = 180 \implies \angle A_1CA_2 + \angle A_{2+n}CA_{3+n}+ \angle A_3CA_4+ \angle A_{4+n}CA_{5+n}+ \ldots \ldots \ldots +\angle A_nCA_{n+1} = 180

[As A i C A j \angle A_iCA_j and A i + n C A j + n \angle A_{i+n}CA_{j+n} are vertically opposite angles when i = j 1 i=j-1 and j n j\leq n ]

180 ( A 1 + A 2 ) + 180 ( A 2 + n + A 3 + n ) + 180 ( A 3 + A 4 ) + 180 ( A 4 + n + A 5 + n ) + + 180 ( A n + A n + 1 ) = 180 \implies 180-( \angle A_1+ \angle A_2) + 180-( \angle A_{2+n}+ \angle A_{3+n}) + 180-( \angle A_3+ \angle A_4)+ 180-(\angle A_{4+n}+A_{5+n})+ \ldots \ldots \ldots +180-(\angle A_n+A_{n+1}) = 180

n × 180 ( A 1 + A 2 + A 2 + n + A 3 + n + A 3 + A 4 + A 4 + n + A 5 + n + + A n + A n + 1 ) = 180 \implies n\times 180-( \angle A_1+ \angle A_2+ \angle A_{2+n}+ \angle A_{3+n} + \angle A_3+ \angle A_4+\angle A_{4+n}+A_{5+n}+ \ldots \ldots \ldots +\angle A_n+A_{n+1} )= 180

180 n F ( n ) = 180 \implies 180n - F(n) = 180 [As, when i j i \neq j , A i + n A_{i+n} and A j + n A_{j+n} are different points]

F ( n ) = 180 ( n 1 ) . \implies F(n) = 180(n-1).

So, F ( 2017 ) = 180 × 2016 = 362880 F(2017)= 180\times 2016 = \boxed{362880} .

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