第一题 是 a²/(a-b) - b²/(a-b) 吧a²/(a-b) - b²/(a-b)= (a²-b²)/(a-b)= (a+b)(a-b)/(a-b)= a+b3x/(x-5) - 2x/(5-x)=3x/(x-5) + 2x/(x-5)= 5x / (x-5)