(from Spherical Trigonometry section)

68.  The Oblique Spherical Triangle.   Let a, b, c denote the sides and α, β, γ the corresponding opposite angles of the spherical triangle, Δ its area, E its spherical excess, R the radius of the sphere upon which the triangle lies, and α ′, β ′, γ ′, a, b, c the corresponding parts of the polar triangle.

0° < a + b + c < 360°,          180° < α + β + γ < 540°.

α = 180° - a ′,       β = 180° - b ′,       γ = 180° - c ′,

a = 180° - α ′,       b = 180° - β ′,       c = 180° - γ ′.

sin α
sin a
  =   sin β
sin b
  =   sin γ
sin c
 .     (Law of Sines).

cos a  = cos b  cos c  + sin b  sin c  cos α .

cos α  = -cos β  cos γ  + sin β  sin γ  cos a.     (Law of Cosines)

tan α
2
  =  
sin (s - b ) · sin (s - c )
sin s · sin (s - a )
,   where s = ½(a + b + c ).

tan a
2
  =  
- cos σ · cos (σ - α )
cos (σ - β ) · cos (σ - γ )
,   where σ = ½(α + β + γ ).

sin ½(α + β )
sin ½(α - β )
  =   tan ½c
tan ½(a - b )
  =   cos ½(α + β )
cos ½(α - β )
  =   tan ½c
tan ½(a + b )

sin ½(a + b )
sin ½(a - b )
  =   ctn ½γ
tan ½(α - β )
  =   cos ½(a + b )
cos ½(a - b )
  =   ctn ½γ
tan ½(α + β )

sin ½(α + β ) cos ½c = cos ½(a - b ) cos ½γ.

cos ½(α + β ) cos ½c = cos ½(a + b ) sin ½γ.

sin ½(α - β ) sin ½c = sin ½(a - b ) cos ½γ.

cos ½(α - β ) sin ½c = sin ½(a + b ) sin ½γ.

tan   E
 4 
  =  
tan ½s · tan ½(s - a ) · tan ½(s - b ) · tan ½(s - c ).

Δ  πR²E 
180
,          α + β + γ - 180° = E .


The information on this page is from HANDBOOK OF MATHEMATICAL TABLES and FORMULAS compiled by Richard Stevens Burington, Ph. D., published by Handbook Publishers, Inc. of Sandusky, Ohio. 1953 edition. (pages 22 and 23)

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