Fungsi f(x) = 2/3x3-1/2x2-2x+5 turunan pada interval

f(x) = ⅔x³ - ½x² - 2x + 5
turun bila f'(x)<0
2x² - x - 2 < 0
–––––––––– : 2
x² - ½x < 1
x² - ½x + 1/16 < 1 + 1/16
(x - ¼)² < 17/16
x - ¼ < +- √17 / 4

u/ x - ¼ = √17 / 4
x = (√17 + 1)/4

u/ x - ¼ = - √17 /4
x = (1 - √17)/4

HP = { (1 - √17)/4 < x < (√17 + 1)/4 }