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°