求函数y=(x-1)x^(2/3)的凹区间
问题描述：
求函数y=(x-1)x^(2/3)的凹区间
答
y'=x^(2/3)+(2/3)(x-1)x^(-1/3)=(5x-1)/[3x^(-1/3)]
y''=10x^(-1/3)/9+2x^(-4/3)/9=[2x^(-1/3)(5+1/x)]/9
当x0,函数是凹的
当-1/5