Question

В треугольнике ABC угол C-прямой, sin B=2/5, AB=18. Найдите BC.

Answer 1

sin²∠B + cos²∠B = 1
cos²∠B = 1 - sin²∠B = 1 - (2/5)² = 1 - 4/25 = 21/25
cos∠B = √21/5

cos∠B = BC / AB
BC = AB · cos∠B = 18 · √21/5 = 18√21/5