M(x,y)
k1k2=2
则[(y-0)/(x+1)][(y-0)/(x-1)]=2
y²=2(x+1)(x-1)=2x²-2
2x²-y²=2
斜率不能为0,否则相乘不等于2
所以x²-y²/2=1,y≠0