a²/(a-1)-(a+1)=[a²-(a+1)(a-1)]/(a-1)=(a²-a²+1)/(a-1)=1/(a-1)
[(a-1)/a]平方-a-1(a的平方-2a+1)/a-a-1(-2a+1)/a-1(-2a+1-a)/a(-3a+1)/a-3+1/a
