Answer
\[\int_{0}^{\pi /4}{\int_{\sin x}^{\cos x}f(x,y){dy}dx}\]
Work Step by Step
\[\begin{align}
& \text{Let the region} \\
& R=\left\{ \left( x,y \right):0\le x\le \pi /4,\text{ }\sin x\le y\le \cos x \right\} \\
& \text{Then,} \\
& \int_{0}^{\pi /4}{\int_{\sin x}^{\cos x}f(x,y) {dy}dx} \\
& \text{Graph} \\
\end{align}\]
