1-[(x-y)/(x+2y)]÷[(x+y)(x-y)/(x+2y)²]
=1-[(x-y)/(x+2y)]×[(x+2y)²/(x+y)(x-y)]
=1-(x+2y)/(x+y)
=(x+y-x-2y)/(x+y)
=-y/(x+y)这100%正确吗?嗯，是