5. (a) Show that \(\sin \theta - \sin 3 \theta\) can be expressed in the form \(a \cos b \theta \sin \theta\), where \(a , b\) are integers whose values are to be determined.
(b) Find the mean value of \(y = 2 \cos 2 \theta \sin \theta + 7\) between \(\theta = 1\) and \(\theta = 3\), giving your answer correct to two decimal places.