y = cos 3x Let u = 3x so that y = cos u Now, \(\frac{\delta y}{\delta x} = 3\), \(\frac{\delta y}{\delta x} = -sin u\) By the chain rule, \(\frac{\delta y}{\delta x} = \frac{\delta y}{\delta u} \times \frac{\delta u}{\delta x}\) \(\frac{\delta y}{\delta x} = (-\sin u) (3)\) \(\frac{\delta y}{\delta x} = -3 \sin u\) \(\frac{\delta y}{\delta x} = -3 \sin 3x\)