（a-b)/（a+2b)除以（a²-b²）/（a²+4ab+4b²）-1
=（a-b)/（a+2b)乘以（a²+4ab+4b²）/（a²-b²）-1
=(a+2b)/(a+b)-1
=b/(a+b)a=-2 b=-1 求值a=-2 b=-1 b/(a+b) =1/3