How to calculate angles

It's more a math problem than a BlitzMax problem, but I need these angles (alpha, beta) to calculate the coordinates at the end of the two yellow lines (light radius) to be able to draw a simple shadow.

The image:

Thanks

tan is your friend here!

http://www.math.com/tables/algebra/functions/trig/overview.htm

Ooh.... I'm stupid. This is so easy, dunno why I didn't realise that.
Thanks anyway!