cosines lawCosinesLaw2001-08-18 21:30:022008-10-01 06:04:20TheoremCosineSineTrigonometryTriangle\usepackage{amssymb}
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\usepackage{xypic}\textbf{Cosines Law.}\, Let $a$, $b$, $c$ be the sides of a triangle and $A$ the angle opposite to $a$. Then
$$a^2 = b^2+c^2-2bc\cos A.$$
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\textbf{Remark.}\, Cosines law is the generalised form of Pythagorean theorem, which latter concerns only the right triangles.