Hi, i got this homework problem i just can't figure out. I'm was given the formula S= sqrt((LD^2)/(L+D)). I must find out how that formula was derived using the basic trig functions (sin,cos,tan, Pythagorean theory, etc...). We where given the diagram below to try and figure this problem out.

http://i39.tinypic.com/312d2l2.jpg

any help you guys can give me will be a great help.