（1+a-1分之1）除（1+a的平方-1分之1）
=[1+1/(a-1)]÷[1+1/(a+1)(a-1)]
=a/(a-1)÷a²/(a+1)(a-1)
=a(a+1)/(a+1)(a-1)x(a+1)(a-1)/a²
=(a+1)/a