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.