1997 AMC 12 Problem 27

Attempt Problem 27 of the 1997 AMC 12 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1997 AMC 12 solutions, or check the answer key.

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27.

Consider those functions ff that satisfy f(x+4)+f(x−4)=f(x)f(x+4)+f(x-4)=f(x) for all real x.x. Any such function is periodic, and there is a least common positive period pp for all of them. Find p.p.
88
1212
1616
2424
3232
Answer: D
Concepts:functional equationsrecurrencesperiodicity
Small hint:
For fixed xx, study the sequence un=f(x+4n)u_n=f(x+4n)
Big hint:
Use un+1+un−1=unu_{n+1}+u_{n-1}=u_n repeatedly to find when every initial pair returns
Solution:
For un=f(x+4n),u_n=f(x+4n), the equation is un+1=un−un−1.u_{n+1}=u_n-u_{n-1}. Starting from u0,u1,u_0,u_1, the sequence is u0,u1,u1−u0,−u0,−u1,u0−u1,u0,u1,…, \begin{gathered} u_0,u_1,u_1-u_0,-u_0,\\ -u_1,u_0-u_1,u_0,u_1,\ldots, \end{gathered} so every such function has period 6⋅4=24.6\cdot4=24. This is least because f(x)=sin⁡(πx12)f(x)=\sin(\frac{\pi x}{12}) satisfies the equation and has fundamental period 24.24. Hence D is correct.
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