x²/9+y²=1y²=1-x²/9F1(-√5,0),F2(√5,0),P(x,y)PF1向量=(-√5-x,-y),PF2向量=(√5-x,-y),PF1向量×PF2向量=x^2+y^2-5=x^2+1-x²/9-5=8/9*x²-4-a