y'=(((x+1)^2)'*3(x-5)^2-(x+1)^2*(3(x-5)^2)')/((3(x-5)^2)^2)=(-4(x+1))/((x-5)^3)y'＞0→-1＜x＜5y'＜0→x＜-1或x＞5(-∞,-1)减(-1,5)增在x=1处取极大值 f(-1)=0
