方程为x^2/4+y^2/3=1
1-x²/4=y²/3
4-x²=4y²/3
设 P(x,y)
A(-2,0),B(2,0)
PA斜率 y/(x+2)
PB斜率 y/(x-2)
斜率之积=y²/(x²-4)=y²/(-4y²/3)=-3/4