Originally Posted by
Pete
Can someone please explain to me how the first derivative of x²(72-4x) is 12x(12-x). I am not sure how to get that answer.
take the derivative of the individual pieces.
dy/dx(72x^2 - 4x^3)= dy/dx(72x^2)-dy/dx(4x^3)
=144x - 12x^2 => 12x(12-x)
If you don't want to open up the parenthesis then you can use the rule:
f*g' + g*f' = (fg)'
so:
f= x^2, f'=2x
g=72-4x, g'=-4
you get:
x^2*(-4)+(72-4x)(2x) = 12x(12-x)