The projection formula states a = b cosC + c cosB, with analogous expressions for b and c. This says that a side equals the sum of the projections of the other two sides onto it. Derived by expanding b cosC + c cosB using the cosine rule. Napier's analogy (tangent rule): tan$\frac{(A-B}{2}$) = $\frac{(a-b}{(a+b)}$) * cot$\frac{C}{2}$. This is used when two sides and their included angle are known, to find the difference of the remaining two angles. Combined with A + B = pi - C, you can solve for both A and B individually. Though less commonly tested, it provides an elegant alternative to the cosine rule for certain configurations.