P(x,y),F1(-c,0)
PF1^2=(x+c)^2+y^2
因为x^2/100+y^2/b^2=1,代入上式,则PF1^2=x^2+2cx+c^2+b^2(1-x^2/100)=...=(10+cx/10)^2
PF1=10+cx/10
同理PF2=10-cx/10
PF1*PF2=100-c^2x^2/100
当x=0时有最大值,100