# Resultant Gamma after Einstein velocity addition

Hello. Let's say that I am in a frame in which I see a rocket travelling at v. This rocket then fires a projectile forward with velocity u in its rest frame.

I can find u' easily enough using the Einstein velocity addition formula. However, I recall seeing a version of the formula that uses gammas instead(ie, if \gamma is defined for v and \Gamma is defined for u, then there is some compact equation for \Gamma' purely as a function of the other two gammas)

Does anyone know this formula? It is proving very difficult to derive. Thanks very much.