{（a-b）/（a+2b）}/{（a²-b²）/（a²+4ab+4b²）}-1={（a-b）/（a+2b）}/{（a+b）（a-b）/（a+2b）²}-1=（a-b）（a+2b）²/（a+b）（a-b）（a+2b）²-1=(a+2b)/(a+b）-1=b/(a+b)...