Essential University Physics: Volume 1 (3rd Edition)

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.