Chapter 7.5, Problem 90E

Rational functions of trigonometric functions An integrand with trigonometric functions in the numerator and denominator can often be converted to a rational integrand using the substitution u = tan (x/2) or equivalently x = 2 tan⁻¹ u. The following relations are used in making this change of variables.

$A:dx=\frac{2}{1+u^2}duB:\sin x=\frac{2u}{1+u^2}C:\cos x=\frac{1-u^2}{1+u^2}$

90. Evaluate ${\displaystyle \int \frac{dx}{2+\cos x}}$.