1. The problem statement, all variables and given/known data http://img20.imageshack.us/img20/112/41590752.jpg [Broken] 2. Relevant equations 3. The attempt at a solution I have no idea how to convert the left equation into the right one. Could someone show me how to do that? I don't understand why the right equation should be multiplied by 1/2 It is really giving me a headache. Thanks for reading this post.