Trigonometry
r=31cos2θr=\frac{31}{\cos^2\theta}r=cos2θ31
r=31sinθcos2θr=\frac{31\sin\theta}{\cos^2\theta}r=cos2θ31sinθ
r=31cosθsin2θr=\frac{31\cos\theta}{\sin^2\theta}r=sin2θ31cosθ
r=31sinθcosθr=\frac{31\sin\theta}{\cos\theta}r=cosθ31sinθ