椭圆x2/9+y2/2=1,焦点F1F2,点P在椭圆上,若P和F1之间距离是4

问题描述:

椭圆x2/9+y2/2=1,焦点F1F2,点P在椭圆上,若P和F1之间距离是4
角F1PF2大小为?

x^2/9+y^2/2=1
PF1+PF2=2a=6
PF1=4,PF2=2
cos∠F1PF2=[(PF1)^2+(PF2)^2-(2c)^2]/2PF1PF2
=(16+4-28)/2*4*2
=-1/2
所以∠F1PF2=120°