(X+1/X)^2=8 =（x-1/x)^2-4X*1/X =（x-1/x)^2-4所以(X-1/X)^2=12 X-1/X=±2倍根下3
(X+1/X)^2=8 =（x-1/x)^2+4X*1/X =（x-1/x)^2+4所以(X-1/X)^2=4 X-1/X=±2
