原式=1-[(a-b)/(a+2b)]÷[(a+b)(a-b)/(a+2b)²]
=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)
=-1/(2+1)
=-1/3