Answer :

[tex]\frac{1}{sec^2\theta}=cos^2\theta\\\\L=\frac{1}{\left(\frac{1}{cos\theta}\right)^2}=\frac{1}{\frac{1}{cos^2\theta}}=1:\frac{1}{cos^2\theta}=1\cdot\frac{cos^2\theta}{1}=cos^2\theta=R[/tex]