woah, some people overcomplicated the solution. Again, if you haven't solved it, dont look.
\frac{\pi r^2}{0.5ab}=2\pi \\ \frac{r^2}{ab}=1\\ r^2=ab\\ but~a=2rsin(\theta ),~and~b=2rcos(\theta)\\ \therefore r^2=4r^2sin(\theta )cos(\theta )\\ 1=2sin(2\theta )\\ sin(2\theta )=\frac{1}{2}\\ 2\theta...