Teorema cosinusului — Teorie

Teorema cosinusului:

$a^{2} = b^{2} + c^{2} - 2 b c \cdot \cos A b^{2} = a^{2} + c^{2} - 2 a c \cdot \cos B c^{2} = a^{2} + b^{2} - 2 a b \cdot \cos C .$

Consecință.

$\cos A = \frac{b^{2} + c^{2} - a^{2}}{2 b c} \cos B = \frac{a^{2} + c^{2} - b^{2}}{2 a c} \cos C = \frac{a^{2} + b^{2} - c^{2}}{2 a b} .$