cosA=(c²+b²-a²)/2bc
$\cos{A} = \dfrac{b^2 + c^2 - a^2}{2bc}$
$\cos{B} = \dfrac{a^2 + c^2 - b^2}{2ac}$
$\cos{C} = \dfrac{a^2 + b^2 - c^2}{2ab}$
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