Relations between sides and angles of any plane triangle
In a plane triangle with angles A,B, and C and sides opposite a,b, and c respectively,
a ⁄ sinA = b ⁄ sinB = c ⁄ sinC = diameter of circumscribed circle.
a2 = b2 + c2 - 2bc cosA
a = b cosC + c cosB
cosA = (b2 + c2 − a2) ⁄ 2bc
tan(A-B ⁄ 2) = (a-b) ⁄ (a+b) cot C ⁄ 2
area = ½ ab sinC = ½ bc sinA = ½ ca sinB = √ s(s-a)(s-b)(s-c) , where s=½ (a+b+c)
Relations between sides and angles of any spherical triangle
In a spherical triangle with angles A,B, and C and sides opposite a,b, and c respectively,
sina ⁄ sinA = sinb ⁄ sinB = sinc ⁄ sinC
cosa = cos b cos c + sin b sin c cos A
cosA = -cos B cos C + sinB sin C cos a
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