Chapter 5 - Review - Exercises - Page 429: 26

$\arctan x + C$

$$\eqalign{ & \int {\frac{{dx}}{{{x^2} + 1}}} \cr & {\text{From the basic derivatives: }}\frac{d}{{dx}}\left[ {\arctan x} \right] = \frac{1}{{{x^2} + 1}},{\text{ then}} \cr & \int {\frac{{dx}}{{{x^2} + 1}}} = \arctan x + C \cr} $$