Вычислите: - 2, 5 ∙ (- 1, 6) + 41, 6 : (-40) 1) 3, 6 2) – 4, 04 3) 4, 04 4) 2, 96

Пошаговое объяснение:

                                  Стеллерова или морская корова!The black book of extinct animals and describes another species, discovered during the expedition of Vitus Bering in 1741. His ship called the "St. Peter" was wrecked near the coast of the island, later named after its discoverer. The team was forced to stay here for the winter and to eat unusual animals called cows due to the fact that they were eating sea grass. These creatures were huge and slow. Their weight often reaches ten tons. The meat of the sea cow was very tasty and useful. Hunting on these harmless giants had not been difficult, as the animals calmly eating algae close to the coast, were not able to escape in the depth and wasn't afraid of the man. In the end, after the expedition to the Islands of profit brutal hunters who killed all the population of the sea cows for some three decades

Х - собственная скорость катера
найдем время катера в движении = 20 час 30 мин = 12 час 30 мин - 160 мин =   8 час - 2 час 40 мин = 5 час 20 мин = 5 1/3 час , из условия задачи имеем :
30 /(х - 3) = 30/ (х + 3) = 5 1/3       30/(х - 3)  + 30 /(х + 3) = 16 / 3    Умножим левую и правую часть уравнения на 3*(х^2 -9) , получим :
30(х + 3) *3 + 30 (х -3)*3 = 16(х^2 -9)      90х + 270 + 90х -270 = 16х^2 -144
180х = 16х^2 -144     45х = 4х^2 - 36      4х^2 - 45х -36 = 0 . Найдем дискриминант уравнения = (-45)^2 - 4*4 *(-36) = 2025 + 576 = 2601
 Найдем sqrt (2601) = 51 . Найдем корни уравнения : 1 -ый = (-(-45)+51) / 2*4 =
(45 +51) / 8  = 96/8 = 12   ; 2-ой = (-(-45) - 51) / 2 * 4 = (45 - 51) / 8 = - 6/8 . Второй корень не подходит . Он меньше 0 , а скорость не может быть <0  .
Значит собственная скорость катера равна = 12 км/ч