t=根号a
t+1/(t-1)^2-4t^2+4t /（t^2-1）^2
t+1/(t-1)^2- 4t^2+4t/(t+1) ^2(t-1)^2
(t+1)^3/(t+1) ^2(t-1)^2- 4t^2+4t/(t+1) ^2(t-1)^2
(t+1)^3-4t^2-4t/(t+1) ^2(t-1)^2
t^3-t^2-t+1/(t+1) ^2(t-1)^2
(t-1)(t^2-1)/(t+1) ^2(t-1)^2
1/t+1
1/根号a+1
答案：根号a-1/a-1