将y=k(x+1)代入y^2=-x,
设A（X1,y1）,B(X2,y2)
易得X1+X2=-(2k^2+1)/k^2,X1*X2=1
y1*y2=k^2(X1+1)(X2+1)=-1
0A斜率K1为y1/X1,0B斜率K2为y2/X2,
所以K1*K2=-1
得证