x^2/4+y^2=1
a^2=4,b^2=1,c^2=4-1=3
共焦点,则设双曲线方程是x^2/a^2-y^2/(3-a^2)=1
Q(2,1)代入得:4/a^2-1/(3-a^2)=1
得a^2=2
即方程是x^2/2-y^2=1