\[\int^{b}_{a}\int^{\phi_{2}(x)}_{\phi_{1}(x)}f(x,y)\,\mathrm{d}y\,\mathrm{d}x=%
\int^{d}_{c}\int^{\psi_{2}(y)}_{\psi_{1}(y)}f(x,y)\,\mathrm{d}x\,\mathrm{d}y.\]