原式=1-(a-b)/(a+2b)×(a²+4ab+4b²)/(a²-b²)
=1-(a-b)/(a+2b)×(a+2b)²/(a+b)(a-b)
=1-(a+2b)/(a+b)
=(a+b-a-2b)/(a+b)
=-b/(a+b)