设半焦距为 c ,P(x,y)
椭圆X2/M2 +y2 =1(m>1)和双曲线 x2/n2 -y2=1(n>1)有相同焦点F1 、F2
M^2-1=C^2
N^2+1=C^2
化得 :M^2+N^2=2C^2
M^2-N^2=2
X2/M2 +y2 =1
x2/n2 -y2=1
得:(M^2+N^2)*y^2=M^2-N^2
y^2=1/C^2
S(PF1F2)=1/2*(2c)*|y|
=1