x-1/x=2
两边平方
x^2-2+1/x^2=4
x^2+1/x^2=6
x^2+2+1/x^2=8
所以(x+1/x)^2=8
所以x+1/x=±2√2
所以x^2-1/x^2
=(x+1/x)(x-1/x)
=±4√2