Tentukan interval f(x) = x (x-6)² + 13 turun

interval fungsi turun f' (x) < 0

f(x) = x(x-6)² + 13 = x(x²-12x+36) + 13
f(x)= x³ -12x²+ 36x + 13

f'(x) < 0
3x² -24x + 36 < 0
x² - 8x + 12 < 0
(x - 6)(x-2) < 0
x > 2  atau x < 6
interval turun 2 < x < 6