$\huge \begin{align} \int \frac{1}{1+x^2}\d x &= \arctan x\\ \int \frac{1}{\sqrt{1-x^2}} \d x &= \arcsin x\\ \int \frac{1}{x\sqrt{1-x^2}} \d x &= \sec^{-1}x \end{align} $ ## Related Formulas $\huge \int \frac{\d x}{a^2 + x^2} = \frac{\arctan \frac{x}{a}}{a} + C $