정답
$\frac{x^2}{a^2} + \frac{y^2}{b^2} = 1$ ($a > b > 0$), with focal distance $c = \sqrt{a^2 - b^2}$ and foci at $(\pm c, 0)$.
해설
The sum of distances from any point on the ellipse to the two foci is constant and equals $2a$. The eccentricity is $e = c/a \in (0, 1)$.