$$\int_{0}^{9}\sqrt{a1-x^{2}}dx$$
$\int _{0}^{9}\sqrt{a_{1}-x^{2}}\mathrm{d}x$
$\frac{\mathrm{d}}{\mathrm{d}a_{1}}(\int _{0}^{9}\sqrt{a_{1}-x^{2}}\mathrm{d}x)$
