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