Odd, I don't even know whether it's neither or not

Algebra Level 4

f ( x ) = x e x 1 + x 2 + 1 \large f(x) = \frac{x}{e^x -1} + \frac{x}{2} + 1

Classify the function above.

Odd Even Neither even nor odd Both even and odd

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1 solution

Chew-Seong Cheong
Oct 19, 2018

Given the function:

f ( x ) = x e x 1 + x 2 + 1 Replacing x with x f ( x ) = x e x 1 x 2 + 1 = x e x 1 e x x 2 + 1 = x e x e x 1 x 2 + 1 = x e x x + x e x 1 x 2 + 1 = x + x e x 1 x 2 + 1 = x e x 1 + x 2 + 1 \begin{aligned} f(x) & = \frac x{e^x-1} + \frac x2 + 1 & \small \color{#3D99F6} \text{Replacing }x \text{ with }-x \\ f(-x) & = \frac {-x}{e^{-x}-1} - \frac x2 + 1 \\ & = \frac {-xe^x}{1-e^x} - \frac x2 + 1 \\ & = \frac {xe^x}{e^x-1} - \frac x2 + 1 \\ & = \frac {xe^x-x+x}{e^x-1} - \frac x2 + 1 \\ & = x + \frac x{e^x-1} - \frac x2 + 1 \\ & = \frac x{e^x-1} + \frac x2 + 1 \end{aligned}

This implies that f ( x ) = f ( x ) f(-x) = f(x) , there f ( x ) f(x) is even ,

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