f(x)=-x^3 +ax^2+ bx
a=0
f(x)=-x^3 + bx
f`(x)=-3x^2+ b=0
x=√b/3 x=-√b/3
（负无穷,-√b/3 ）与（√b/3,正无穷）为减
（-√b/3,√b/3）为增
f(x)在x=-2处取得极小值
f`(x)=-3*4+ b=0
b=12
f(x)=-x^3 +ax^2+ bx
a=0
f(x)=-x^3 + bx
f`(x)=-3x^2+ b=0
x=√b/3 x=-√b/3
（负无穷,-√b/3 ）与（√b/3,正无穷）为减
（-√b/3,√b/3）为增
f(x)在x=-2处取得极小值
f`(x)=-3*4+ b=0
b=12