Question

Find the values of a and b if the function y= 2x^3+ax^2+bx+36 has a local maximum when x=-4 and a local minimum of x=5

Answer 1

I know y' = 6x^2 + 2ax +b

but then I was keep subbing in x=-4 only
then what is the x=5 for?

Answer 2

these are the x values when y' = 0. you'll need both of them to find a and b.

Answer 3

yes I put y' as 0, and then sub in x=4
but i got b= -30 and a = 45/4

my friend got it right, but he uses x=5 as well..but i dont know why

Answer 4

so,