Question

AB=2cm, AC=3cm, BAC=60' BC=??

Answer 1

. По теореме косинусов:

BC² = AC² + AB² - 2AC•AB•cosBAC

BC² = 4 + 9 - 2•2•3•1/2 = 13 - 6 = 7 => BC = √7.

2. По теореме косинусов:

BC² = AC² + AB² - 2AC•AB•cosА

Откуда cosA = (AB² + AC² - BC²)/(2•AB•AC)

cosA =(49 + 36 - 25)/(7•6•2) = 60/ 84 = 15/ 21 = 5/7