This is not a problem statement this is not homework this is not a textbook exercise. This is my own question about a formula in a textbook. 1. The problem statement, all variables and given/known data I am trying to understand the way that equation 4.2.2 is rewritten as equation 4.2.3 2. Relevant equations Source: Stanley Middleman 3. The attempt at a solution My confusion is with the 1/2 and the subscripts used.