因?yàn)閨OP|=根號(hào)7乘a ,設(shè)P(x,y) x^2+y^2=7a^2 x^2/a^2--y^2/b^2=1
y^2=6(ab)^2/(b^2+a^2)=6(ab)^2/c^2
因?yàn)椤螰1PF2=60° 設(shè)F1P與X軸夾角為a1 tana1=y/(x+c)
設(shè)F2P與X軸夾角為a2 tana2=y/(x--c) a2--a1=60
tan60=根3=tan(a2--a1)=[y/(x--c)-y/(x+c)]/[1+y/(x--c)*y/(x+c)]=2yc/(x^2+y^2--c^2)=
根3=2yc/(7a^2--c^2) y=根3(7a^2--c^2)/2c y^2=3(7a^2--c^2)^2/4c^2=6(ab)^2/c^2
(6a^2--b^2)^2=8(ab)^2 6a^2--b^2=2根2ab b=√2a b=--3v2a