Hint on Exercise E4.4 (b), Runge-Lenz vector

[Jay Wang]

Let "x" be the cross product, the derivative of p x L produces two terms, (F+f) x L and p x [r x (F+f)]; leave the 3rd term (m k r^hat) alone. From part (a), you know F x L + p x (r x F) - d(3rd term)/dt =0. This leads to the required answer.