f(x)=2x^3-6x^2+ 3
f'(x) = 6x^2 - 12x =0
x(x-2) =0
x = 0 or 2
f''(x) = 12x-12
f''(0) = -12 ( max)
f''(2) = 12 ( min)
f(0) = 3 > 0
f(2) = 2(8)-6(4) + 3
= -5 < 0
f(x) is increasing on （-∞,0]
from -ve -> +ve
f(x) is decreasing on [0,2]
from +ve -> -ve
f(x) is increasing on [2,∞)
from -ve -> +ve
f(-∞) = -∞
一个根属于（-∞,0]
一个根属于[0,2]
一个根属于[2,∞)
total:3 real roots