I now make use of the sum of the sum-and-difference formulas from page 38 of chapter 3, namely cosAcosB=12(cosS+cosD) and sinAsinB=12(cosD−cosS),...I now make use of the sum of the sum-and-difference formulas from page 38 of chapter 3, namely cosAcosB=12(cosS+cosD) and sinAsinB=12(cosD−cosS), to obtain In these Equations we already know an approximate value for f3 (we’ll see how when we resume our numerical example); the unknowns in these Equations are R3, a and g3, and it is R3 that we are trying to find.