a/b(a-b)-b/a(a-b)；=[a²(a-b)-b²(a-b)]/ab(a-b)=(a-b)²(a+b)/ab(a-b)=(a²-b²)/ab（1/a+1/b）÷a²+2ab+b²/a+b.=(a+b)ab × (a+b)/(a+b)²=ab