 But a' = 180° - a, c' = 180° - c, and CAB' = 180° - A Hence cos (180° — a) =cos b cos (180° - c) + sin b sin (180° - c) cos (180a- A)9 or, cos a = cos b cos с -f- sin b sin с cos A, which- proves the law of cosines for all cases.  Plane and Spherical Trigonometry - Page 97by Leonard Magruder Passano - 1918 - 144 pagesFull view - About this book
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