Here's the question you clicked on:
arcticf0x
If the side and angle of a plane triangle ABC vary in such a way that the circumradius remains constant, then prove that:
\[\delta a / \cos A + \delta b / \cos B + \delta c / \cos C = 0\] where da, db and dc are samll increments in sides a,b,c respectively.
I guess not a good time to post it. RIP!