Assume that A 1 A n + 1 , A 2 A n + 2 , A 3 A n + 3 , … … … and A n A 2 n are n ≥ 3 concurrent lines, where n is an odd positive integer. The point of concurrence is C .
A counter-clockwise rotating ray with its endpoint fixed at C and initial position along C A 1 passes, in its first full rotation, A i before A j whenever i < j .
Draw the segments A 1 A 2 , A 3 A 4 , A 5 A 6 … … … and A 2 n − 1 A 2 n .
Now we define the following function 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 ) .
The following figure illustrates the case for n = 3 with the defining angles for F ( 3 ) indicated.
Find F ( 2 0 1 7 ) , in degrees.
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First notice, ∠ C A i A j contributes to F if and only if i − j = ± 1 and the minimum of i and j is an odd integer.
Now, we have, in degree measure, ∠ A 1 C A n + 1 = 1 8 0
[As 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 = 1 8 0
⟹ ∠ 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 = 1 8 0
[As ∠ A i C A j and ∠ A i + n C A j + n are vertically opposite angles when i = j − 1 and j ≤ n ]
⟹ 1 8 0 − ( ∠ A 1 + ∠ A 2 ) + 1 8 0 − ( ∠ A 2 + n + ∠ A 3 + n ) + 1 8 0 − ( ∠ A 3 + ∠ A 4 ) + 1 8 0 − ( ∠ A 4 + n + A 5 + n ) + … … … + 1 8 0 − ( ∠ A n + A n + 1 ) = 1 8 0
⟹ n × 1 8 0 − ( ∠ 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 ) = 1 8 0
⟹ 1 8 0 n − F ( n ) = 1 8 0 [As, when i = j , A i + n and A j + n are different points]
⟹ F ( n ) = 1 8 0 ( n − 1 ) .