Velocity in Lorentz Transformations

[zimbabwe]

I'm reviewing for exams and don't understand when to use which Lorentz velocity equation to use.
one goes

v'=(v-u)/(1-vu/c^2)

and the second

v=(v'+u)/(1+v'u/c^2)

 

[AEM]

You should probably let us know some details such as how your coordinate systems are labeled i.e. is the unprimed system system 'stationary' or moving, and which direction the moving system is going in. Common usage could be assumed, but I'd prefer not to make that assumption.

 

[turin]

It looks like you have a particle moving at velocity, v, in some frame of reference, K. Then, you consider another frame of reference, K', moving at velocity, u. Then, the equations tell you the velocity of the particle, v', as seen in K' in terms of v, and the velocity of the particle, v, as seen in K in terms of v'.

 

[zimbabwe]

hmm ok here's the question

two particles move in opposite directions, with one particle at a speed .784c and the other 0.650c as measured by the laboratory. What is the speed of one particle relative to the other.

To get the right answer it's the second equation= .95c

v=(v'+u)/(1+v'u/c^2)
v'=.784c u=.650

the first equation gives .375, which doesn't fit anywhere. So i don't see what it would be used for.