Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 4 / 58 / b / v / Solution

Set $\mathbf{k}=\mathbf{h}$ in the direct-methods triplet relationship. It becomes $S(\mathbf{h}{)}^{2}S(2\mathbf{h})\simeq +1$, and since the square of a nonzero sign is one, $S(2\mathbf{h})\simeq +1$. This recovers the direct-methods pair relationship, with the same strong-amplitude qualifications.