Grafik fungsi f(x) = x3 + ax2 + bx + c hanya turun pada interval –1 < x < 5. Nilai a + b =

f(x) = x^3 + ax^2 + bx + c
f'(x) = 3x^2 + 2ax + b
Turun pada interval -1 < x < 5 artinya f'(-1) = 0 dan f'(5) = 0
f'(-1) = 3(-1)^2 + 2a(-1) + b = 0 => 3 - 2a + b = 0
f'(5) = 3(5)^2 + 2a(5) + b = 0 => 75 + 10a + b = 0
------------------------------------------------------------------- --
........................................................... -72 - 12a = 0 => -12a = 72 => a = -6
3 - 2a + b = 0
3 - 2(-6) + b = 0
3 + 12 + b = 0
b = -15

a + b = -6 - 15 = -21