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