（2x+y）^2-(x+2y)^2 平方差公式＝（2x+y+x+2y）（2x+y-x-2y）=(3x+3y)(x-y)=3(x+y)(x-y)
原式=（2x+y+x+2y）（2x+y-x-2y）=(3x+3y)(x-y)=3(x+y)(x-y)
