Chapter 12 - Section 12.3 - Higher Order Derivatives: Acceleration and Concavity - Exercises - Page 901: 8

$e^{-x}+e^{x}$

$f''(x)=\displaystyle \frac{d}{dx}\left[\frac{df}{dx}\right]=\frac{d^{2}f}{dx^{2}}$ $\displaystyle \frac{dy}{dx}=\frac{d}{dx}[e^{-x}+e^{x}]=-e^{-x}+e^{x}$ $\displaystyle \frac{d^{2}y}{dx^{2}}=\frac{d}{dx}[ -e^{-x}+e^{x}]=-(-e^{-x})+e^{x}$ = $e^{-x}+e^{x}$