In the Concise Encyclopedia of Mathematics of Eric W. Weisstein, in its second edition, I came across a formula noted in page $379$ (entry Cevian):
$$ s = \frac{b \sin \alpha '}{\sin(\gamma + \alpha ')}$$
Here is the corresponding image of the Cevian entry of the Encyclopedia:
No demonstration included; so, I tried to understand where it could come from, but I haven't been able to prove this formula.
Actually, I even don't understand where could I start to try to demonstrate it...
Could I have some indications, directions from you?
Much appreciated.