## Calculus: Early Transcendentals 8th Edition

The function $u_{x}=f(x+at)+g(x-at)$ is a solution of the equation $u_{tt}(x,t)=a^2 u_{xx}(x,t)$
We need to take the partial derivative of $u$ with respect to $t$. We have $u_{t}(x,t)=f'(x+at)[\dfrac{\partial }{\partial t}(x+at)]+g'(x-at)[\dfrac{\partial }{\partial t}(x-at)]=af'(x+at)-ag'(x-at)$ Further, $u_{tt}(x,t)=a^2f''(x+at)+a^2g''(x-at)$ Now take the partial derivative of $u$ with respect to $x$. We have: $u_{x}(x,t)=f'(x+at)+g'(x-at)$ Finally, we will take the second derivative of $u$ with respect to $x$. $u_{xx}(x,t)=f''(x+at)+g''(x-at)$ Hence, the result has been verified.