Question

plz solve fast with explain I will give brain mark ​

Answer 1

he derivative of cos3(x) is equal to:

−3cos2(x)⋅sin(x)

You can get this result using the Chain Rulewhich is a formula for computing the derivative of the composition of two or more functions in the form: f(g(x)).

You can see that the function g(x) is nested inside the f() function. 

Deriving you get:

derivative of f(g(x)) --> f'(g(x))⋅g'(x)

In this case the f() function is the cube or ()3while the second function "nested" into the cube is cos(x).

First you deal with the cube deriving it but letting the argument g(x) (i.e. the cos) untouched and then you multiply by the derivative of the nested function. 



Which is equal to: −3cos2(x)⋅sin(x)