Remember that putting a negative sign in front of a function means the same thing as multiplying that function by -1.

Sample Question

Let g(x) = -x². Then we could think of this function as

g(x) = (-1)(x²),

therefore

g'(x) = (-1)(x²)' = (-1)(2x) = -2x.

The moral of the story is that the derivative of the negative of f is the negative of the derivative of f:

(-f(x))' = -f'(x).

Sample Problem

Let f(x) = -sin x. Then 

f'(x) = -(sin x)' = -cos(x). 

Practice: