9(x+y)的平方—4（x--y)的平方=[3(x+y)+2(x-y)][3(x+y)-2(x-y)]=(5x+y)(x+5y)
原式=[3(x+y)]^2-[2(x-y)]^2=[3(x+y)+2(x-y)][3(x+y)-2(x-y)]=(5x+y)(x+5y)
