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If y = cos 3x, find δyδx...

If y = cos 3x, find δyδx

A.

13sin3x

B.

13sin3x

C.

3 sin 3x

D.

-3 sin 3x

Correct answer is D

y = cos 3x

Let u = 3x so that y = cos u

Now, δyδx=3,

δyδx=sinu

By the chain rule,

δyδx=δyδu×δuδx

δyδx=(sinu)(3)

δyδx=3sinu

δyδx=3sin3x