(i)[3]
Express the equation $\cot \theta - 2\tan \theta = \sin 2\theta$ as $a\cos^4 \theta + b\cos^2 \theta + c = 0$, with constants $a$, $b$ and $c$ to be found.
(ii)[2]
Hence, for $90^\circ < \theta < 180^\circ$, solve the equation $\cot \theta - 2\tan \theta = \sin 2\theta$.