4 Variables \(u , x\) and \(y\) are such that \(u = 2 x ( y - x )\) and \(x + 3 y = 12\). Express \(u\) in terms of \(x\) and hence find the stationary value of \(u\).
- Prove the identity \(\frac { \sin \theta - \cos \theta } { \sin \theta + \cos \theta } \equiv \frac { \tan \theta - 1 } { \tan \theta + 1 }\).
- Hence solve the equation \(\frac { \sin \theta - \cos \theta } { \sin \theta + \cos \theta } = \frac { \tan \theta } { 6 }\), for \(0 ^ { \circ } \leqslant \theta \leqslant 180 ^ { \circ }\).