1981 AHSME Problems/Problem 17

Problem

The function $f$ is not defined for $x=0$ , but, for all non-zero real numbers $x$ , $f(x)+2f\left(\dfrac{1}x\right)=3x$ . The equation $f(x)=f(-x)$ is satisfied by

$\textbf{(A)}\ \text{exactly one real number} \qquad \textbf{(B)}\ \text{exactly two real numbers} \qquad\textbf{(C)}\ \text{no real numbers}\qquad \\ \textbf{(D)}\ \text{infinitely many, but not all, non-zero real numbers} \qquad\textbf{(E)}\ \text{all non-zero real numbers}$

Solution

Substitute $x$ with $\frac{1}{x}$ :

$f\left(\frac{1}{x}\right)+2f(x)=\frac{3}{x}$ .

Adding this to $f(x)+2f\left(\dfrac{1}x\right)=3x$ , we get

$3f(x)+3f\left(\dfrac{1}x\right)=3x+\frac{3}{x}$ , or

$f(x)+f\left(\dfrac{1}x\right)=x+\frac{1}{x}$ .

Subtracting this from $f(\frac{1}{x})+2f(x)=\frac{3}{x}$ , we have

$f(x)=\frac{2}{x}-x$ .

Then, $f(x)=f(-x)$ when $\frac{2}{x}-x=\frac{2}{-x}+x$ or

$\frac{2}{x}-x=0$ , so $x=\pm{\sqrt2}$ are the two real solutions and the answer is $\boxed{(B)}$ .

1981 AHSME (Problems • Answer Key • Resources)
Preceded by Problem 16	Followed by Problem 18
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1981 AHSME Problems/Problem 17

Problem

Solution

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