2x^3+3x^2-120x

Question

to find the critical numbers I took the derivative which is 6x^2+6x-120 so divide by 6 to simplify

x^2+x-20=0

so the critical numbers will be (4,0) and (-5,0) but its wrong I don't know y?

Answer 1

Your work looks good to me, but the critical numbers are 4 and -5, not the points you give.

Recall the definition:

A number a in the domain of a given function f is called a critical number of f if f '(a) = 0 or f' is undefined at x = a.

Answer 2

Thnks :)