Point form: a^2x/x1 - b^2y/y1 = a^2 - b^2 = c^2. Parametric: ax/cos(theta) - by/sin(theta) = a^2 - b^2. Slope form: y = mx - m(a^2-b^2)/sqrt(a^2+b^2m^2). From an external point, at most 4 normals can be drawn (unlike 3 for parabola).