4(x+2y)^2-9(2x-y)^2
=[2(x+2y)-3(2x-y)][2(x+2y)+3(2x-y)]
=(-4x+7y)(8x+y)负号不能顶格写=(-4x+7y)(8x+y)=(7y-4x)(8x+y)你数学谁教的，还有这样的规定？