(3-17)  We have: 

①   Squaring gives us:  

and rearranging gives:   

②   Separating out, we get:    

And substituting in our original expression for E2 on the right hand side, this becomes:

③   Squaring the momentum expression (III-1), we get:

A quick comparison with our expression in ② allows us to insert this into it, to give: