Lorentz Velocity Transformations Question

[zellwwf]

Homework Statement

An electron and a positron are observed from the lab to move in opposite
directions with a speed of 0.5c and 0.7c, respectively. Find the speed of the
positron when observed in the electron's rest frame.

Homework Equations

in attached PDF

The Attempt at a Solution

in attached PDF
---------------------%------------------
I would like you to check my solution, if it is wrong, show me where i went wrong, and give me a hint, if it's right, gimme a cookie!
this forum has been helping me immensely with Modern Physics.

Please refer to the attached Document

 

[voko]

You ended up deriving the velocity addition formula. I wonder why you could not just have used it directly. The result seems correct.

 

[zellwwf]

if i don't derive it a couple of times myself, i won't memorize it, and i won't know "what" it means, ...
i was taught physics 1 by a great professor who didn't use except two formulas (f=ma, and a=+- 9.8) we had to derive everything else..
i remember he said once when we told him it was hard to derive the displacement from the acceleration... he looked at us with such a stare and said "Wow... are you all really that stupid! It's not that hard"...