how to find ellipse focuses

line font diagram text brand circle drawing cartoon document presentation logo wheel number sketch ellipse focuses vector astronomy axis center central symmetry circle circumference closed type conic constant curve line diagonal diameter dimensions directrix draftsmanship eccentricity education ellipse engineering equation focal distance points focus positions form formula geometric center geometry height hyperbola infinite elongation intersection invariant machinery major mathematics minor orbit planet parabola perimeter physics polar radius curvature ratio semi-axis sum width