原式=[(a+2b)(a-b)]/[(a+b)(a-b)]+[2b²/(a+b)(a-b)]=[(a+2b)(a-b)+2b²]/[(a+b)(a-b)]=(a²+ab-2b²+2b²)/[(a+b)(a-b)]=[a(a+b)]/[(a+b)(a-b)]=a/(a-b)