$\huge \int_{0}^{\frac{1}{3}} \frac{\d{x}}{\sqrt{1-x^2}} $ $\huge \begin{align} \int_{0}^{\frac{1}{3}} \frac{\d{x}}{\sqrt{1-x^2}} &= \arcsin \frac{1}{3} - \arcsin 0\\ &= \arcsin \frac{1}{3} \end{align} $