Даны вершины треугольника ABC A (-1; 4; 6) B (-4; 2; 1) C (-1; 2; 5) найти косинус угла ABC

CosABC = (AB*BC) / (|AB|*|BC|)

AB = (-4 - (-1), 2-4,1-6) = (-3,-2,-5)

BC = (-1 - (-4), 2-2,5-1) = (3,0,4)

|AB| = sqrt (9+4+25) = sqrt (38)

|BC| = sqrt (9+0+16) = sqrt (25) = 5

cosABC = (-9+0-20) / (sqrt (38) * sqrt (5)) = - 29 / (sqrt (38) * sqrt (5))