Full question: Prove that the area of the triangle formed by three points of an ellipse, whose eccentric angles are $\theta , \phi$ and $\psi$ , is
My attempt: I found the equation of the line joining the points with eccentric angles $\theta$ and $\phi$. Then found the perpendicular distance H of the point with ecc. angle $\psi$ from this line. Then I found the length D of the interval joining the first two points.
So the area is 1/2 DH, however I am unable to simply this expression into the form required.