Angles in an ellipse proofs

[fled314re]

Hello,

Could you please help me with the following problem?

"Let A be a point on an ellipse with the focal point B1 near vertex S1 and the focal point B2 at vertex S2. Let C be the intersection of the perpendicular from B2 to the tangent through point A. Prove: The angles ACS1 and B2CS2 are equal."

Thanks in advance!