Point form: xx1/$a^2$ + yy1/$b^2$ = 1. Slope form: y = mx +/- sqrt(a^{2m}^2 + $b^2$), with tangent condition $c^2$ = a^{2m}^2 + $b^2$. Parametric: (xcos(theta))/a + (ysin(theta))/b = 1. Two tangents can be drawn from any external point. From a point on the ellipse, exactly one tangent. From an interior point, none.