Other formulas of spherical trigonometry
 

1. If s = (a + b + c)/2,

 

Obviously there are two variations of this, for sin B/2 and sinC/2.
 

2. By writing 180 − a for A, 180 − b for B, 180 − c for C, 180 − A for a, etc, in the four main
formulas, other useful formulas can be obtained. For example, using the cosine formula (1),

cos a = cos b cos c + sin b sin c cos A.......(1)

we obtain
 

cos A = −cos B cosC + sin B sin C cos a.
 

Proofs of these formulas will not be given here.