derivative 1

Question

Find the derivative of f(x) = -2x² + 11x at x = 9.

Answer 1

f(x) = -2x² + 11x

Take derivative both sides with respect to x

f'(x) =d/dx(-2x² + 11x)

    =-2 d/dx(x² ) +11 d/dx(x)

   =-2(2x)+11

   =-4x+11

At x = 9

f'(9) = -4(9)+11 =-36+11 =-25

f'(9)= -25