设P(x1，y1) 左右顶点为A(-a，o) B(a，o)
K1=y1/(x1+a) K2=y2/(x1-a)
k1k2=y1^2/(x^2-a^2)
p在椭圆上则x1^2/a^2+y1^2/b^2=1 即y1^2=b^2-b^2X1^2/a^2=(b^2/a^2)(a^2-x1^2)
代入得k1k2=-b^2/a^2
