a是方程x的平方-x=6的根
∴a²-a=6
(a-2)/(a+1)(a-1)÷(a²-1-2a+1)/(a+1)
=(a-2)/(a+1)(a-1)×(a+1)/a(a-2)
=1/a(a-1)
=1/(a²-a)
=1/6