Show that an equation of the chord joining the points [latex] P(acos\omega, bsin\omega) and Q(acos\theta, b sin\theta) [/latex] on the ellipse with equation [latex] b^2x^2+a^2y^2=a^2b^2 is

Bxcos\frac{1}{2}(\theta+\omega)+aysin\frac{1}{2}(\ theta +\omega)=abcos\frac{1}{2}(\theta-\omega) [/latex]

I just can't seem to make it work. Help please!!!