Solution
4.
[tex]\begin{gathered} \frac{cos\theta}{1+sin\theta}+\frac{cos\theta}{1-sin\theta}=\frac{cos\theta(1-sin\theta)+cos\theta(1+sin\theta)}{(1+sin\theta)(1-sin\theta)} \\ \\ \frac{cos\theta}{1+sin\theta}+\frac{cos\theta}{1-sin\theta}=\frac{cos\theta-sin\theta cos\theta+cos\theta+sin\theta cos\theta}{1-sin^2\theta} \\ \\ \frac{cos\theta}{1+sin\theta}+\frac{cos\theta}{1-sin\theta}=\frac{2cos\theta}{cos^2\theta} \\ \\ \frac{cos\theta}{1+sin\theta}+\frac{cos\theta}{1-sin\theta}=\frac{2}{cos\theta} \\ \\ \frac{cos\theta}{1+sin\theta}+\frac{cos\theta}{1-sin\theta}=2sec\theta \end{gathered}[/tex]
