Answer
The proof is below.
Work Step by Step
We take the second order partial derivatives with respect to x and t and plug them into equation 14.5 to find:
$f''(x\pm vt)=\frac{1}{v^2}f''(x\pm vt)\times v^2$
$f''(x\pm vt)=f''(x\pm vt)$
Thus, the proof is complete.
