y=x^(5/3)-x^(2/3)y''=(5/3)(2/3)x^(-1/3)-(2/3)(-1/3)x^(-4/3)=(10x+2)/9x^(4/3)拐点x=-1/5和x=0x＜-1/5或x＞0时y''＞0为凹趋近-1/5＜x＜0为凸区间
