Chord Length on an Ellipse

Question

Let the points P and Q be on the ellipse x^2/a^2 + y^2/b^2 = 1 where P(x,y) = (acos(θ), bsin(θ)) and Q(x,y) = (acos(φ), bsin(φ)).

Show that the equation of the chord PQ is (x/a)cos[(θ+φ)/2] + (y/b)sin[(θ+φ)/2] = cos[(θ-φ)/2]

Answer 1