1983 AHSME Problems/Problem 18

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Problem: Let $f$ be a polynomial function such that, for all real $x$, \[f(x^2 + 1) = x^4 + 5x^2 + 3.\] For all real $x$, $f(x^2 - 1)$ is

Solution: (A) $x^4 + 5x^2 + 1$ (B) $x^4 + x^2 - 3$ (C) $x^4 - 5x^2 + 1$ (D) $x^4 + x^2 + 3$ (E) none of these Let $y = x^2 + 1$. Then $x^2 = y - 1$, so we can write the given equation as f(y)=x4+5x2+3=(x2)2+5x2+3=(y1)2+5(y1)+3=y22y+1+5y5+3=y2+3y1. Then substituting $x^2 - 1$, we get f(x21)=(x21)2+3(x21)1=x42x2+1+3x231=x4+x23. The answer is (B).