设y=kx-1代入x^2-y^2=1 得(1-k^2)x^2+2kx-2=0
根号(8-4k^2)=2根号2绝对值(1-k^2)
8-4k^2=8(1-k^2)^2
2k^4-3k^2=0
k^2=3/2
k=正负2分之根号6