计算或化简：[（x-y）/(x+3y)]÷[（x²-y²）/(x²+6xy+9y²)]-（2y）/x+y
问题描述：
计算或化简：[（x-y）/(x+3y)]÷[（x²-y²）/(x²+6xy+9y²)]-（2y）/x+y
答
原式=[(x-y)/(x+3y)]×[(x+3y)²/(x+y)(x-y)]-2y/(x+y)
=(x+3y)/(x+y)-2y/(x+y)
=(x+3y-2y)/(x+y)
=(x+y)/(x+y)
=1