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2007 AIME II Problems/Problem 14

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Problem

Let be a polynomial with real coefficients such that and for all , \displaystyle f(x)f(2x^{2})=f(2x^{3}+x). Find

Solution

Note:The following solution(s) are non-rigorous.

Substitute the values . We find that \displaystyle f(i)f(-2) = f(-i), and that \displaystyle f(-i)f(-2) = f(i). This means that f(i)f(-i)f(-2)^2 = f(i)f(-i) \Longrightarrow f(i)f(-i)(f(-2) - 1) = 0. This suggests that are roots of the polynomial, and so \displaystyle (x - i)(x + i) = x^2 + 1 will be a root of the polynomial.

The polynomial is likely in the form of ; appears to satisfy the same relation as , so it also probably has the same roots. Thus, is the solution. Guessing values for , try . Checking a couple of values shows that works, and so the solution is .

See also

2007 AIME II (ProblemsResources)
Preceded by
Problem 13
Followed by
Problem 15
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
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