Well, I know at least that I'm looking for dA/d(alpha), which is equal to dA/dr * dr/d(alpha)

dA/dr would be the derivative of the area respect to r, but I'm not sure what the formula for the area of this triangle is.. heheh..

dr/d(alpha) I -think- I can get with the cosine rule, from where I get the relationship arccos((a^2 -2r^2)/(-2r^2)) = alpha.

Derivating that I get dr/da =

So yup, any help would be appreciated.