（a-1分之a+1 + a的平方-2a+1分之1) 除以 a-1分之a=[(a+1)/(a-1)+1/(a-1)²]×(a-1)/a=[(a+1)(a-1)/(a-1)²+1/(a-1)²]×(a-1)/a=a²/(a-1)²×(a-1)/a=a/(a-1)