y=1/x 与 y=x-1 交于(a,b)
1/x = x-1
将 x=a 代入 1/x = x-1 :
1/a = a-1
将 x=a y=b 代入 y=1/x :
b=1/a
1/b=a
1/a - 1/b = (a-1) - a = a-1-a = -1