Chapter 14.3, Problem 37ES

If R is the region in the first quadrant between the circles $r=1\text{and}\text{r}=2,$ and if $f$ is continuous on R, then

${\displaystyle \underset{R}{\iint }f(r,\theta )dA={\displaystyle {\int }_0^{\pi /2}{\displaystyle {\int }_1^{2}f(r,\theta )}}drd\theta }$