此式可以写成:[2(x+y)]^2-[3(x-y)]^2=(2x+2y+3x-3y)(2x+2y-3x+3y)=(5x-y)(5y-x
9(x+y)²-4(x-y)²=[3(x+y)+2(x-y)][3(x+y)-2(x-y)]=(5x+y)(x+5y)
平方差公式啊~~（5x+y）(x+5y)
