(a) Show that \(\dfrac{1-\sin x}{\cos x}+\dfrac{\cos x}{1-\sin x}=2\sec x\).

(b) Hence solve the equation \(\dfrac{1-\sin \frac{\theta}{2}}{\cos \frac{\theta}{2}}+\dfrac{\cos \frac{\theta}{2}}{1-\sin \frac{\theta}{2}}=3\) for \(0^\circ\leqslant \theta\leqslant720^\circ\).