Answer
See below.
Work Step by Step
$LHS=\frac{1-sin\theta}{1+sin\theta}=\frac{1-sin\theta}{1+sin\theta}\times\frac{1-sin\theta}{1-sin\theta}=\frac{(1-sin\theta)^2}{1-sin^2\theta}=\frac{(1-sin\theta)^2}{cos^2\theta}=(\frac{1-sin\theta}{cos\theta})^2=(sec\theta-tan\theta)^2=RHS$
