Question

Calcule o ângulo A do triangulo cujo lados são AB=15cm, AC=9cm e BC=21cm.

Answer 1

Pela lei dos cossenos

${21}^{2} = {15}^{2} + {9}^{2} - 2.15.9. \cos(\theta)$

$441 = 225 + 81 - 270 \cos(\theta)$

$441 - 306 = - 270 \cos(\theta) \\ \cos(\theta) = - \frac{135}{270} \\ cos(\theta) = - \frac{1}{2} \\ \theta = 120°$