在区间〔1,3〕上, f'(x) = x^2 + 2ax + 5 = (x - (-a))^2 + 5 - a^2 >= 0, f"(1)=2a+6, f'(3)=6a+14
==>
(1) -a = 0 a < -3, 6a+14 >= 0 a < -3, a >= -14/6
a的取值范围 a >= -5^(1/2)
在区间〔1,3〕上, f'(x) = x^2 + 2ax + 5 = (x - (-a))^2 + 5 - a^2 >= 0, f"(1)=2a+6, f'(3)=6a+14
==>
(1) -a = 0 a < -3, 6a+14 >= 0 a < -3, a >= -14/6
a的取值范围 a >= -5^(1/2)