If y cos 3x find frac delta y delta x - JAMB Mathematics 2014 Question

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)