First, I rewrite the energy U in terms of P1 and P2. This is necessary so that the partial derivatives may be taken. (When values are given, DO NOT substitute the numbers until after taking the partial derivatives.)
This fits equation 11.61 on page 711:
where the influence coefficients are
To find the deflections at positions 1 and 2, I use the partial derivatives according to Castigliano's Theorem:
