Integrate $f\left(x\,y\,z\right)$ over a General Region

${\int }_{x_1}^{x_2}{\int }_{y_1\left(x\right)}^{y_2\left(x\right)}{\int }_{z_1\left(x\,y\right)}^{z_2\left(x\,y\right)}f \mathit{\ⅆ}z \mathit{\ⅆ}y \mathit{\ⅆ}x$                  $f\left(x\,y\,z\right)\=$      

$\mathrm{dz} \mathrm{dy} \mathrm{dx}$