Thanks for the reply.
Yes I know the formula which involves theta but limits for 'r' is given , not theta. It is from here I faced difficulty.Please help}
Remember that $\displaystyle \displaystyle \frac{dr}{d\theta} = \frac{1}{\frac{d\theta}{dr}}$. I assume you are using $\displaystyle \displaystyle \phi$ to represent $\displaystyle \displaystyle \theta$...