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If y = cos 3x, find \(\frac{\delta y}{\delta x}\)...

If y = cos 3x, find \(\frac{\delta y}{\delta x}\)

A.

\(\frac{1}{3} \sin 3x\)

B.

\(-\frac{1}{3} \sin 3x\)

C.

3 sin 3x

D.

-3 sin 3x

Correct answer is D

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,

\(\frac{\delta y}{\delta x} = \frac{\delta y}{\delta u} \times \frac{\delta u}{\delta x}\)

\(\frac{\delta y}{\delta x} = (-\sin u) (3)\)

\(\frac{\delta y}{\delta x} = -3 \sin u\)

\(\frac{\delta y}{\delta x} = -3 \sin 3x\)