Предмет: Алгебра, автор: marinagrishaeva

Расстояние между пристанями А и В равно 90 км. Из А в В по течению реки отправился плот , а через час вслед за ним отправилась моторная лодка , которая ,прибыв в пункт В , тотчас повернула обратно и возвратилась в А . К этому времени прошел 52 км . Найдите скорость лодки в неподвижной воде , если скорость течения реки , равна 4 км/ч. С пояснением пожалуйста !

Ответы

Автор ответа: Аноним

х - скорость лодки в стоячей воде
х + 4 - скорость по течению
х - 4 скорость лодки против течения
52 4 = 13 часов плыл плот
13 - 1 = 12 часов плыла лодка из А в В и обратно
Составим уравнение
90 ( х+4 ) + 90 ( х - 4 ) = 12
х²- 15 х - 16 = 0
Дискриминант = 225 + 4 * 1 * 16 = 289 = 17
х = (15 + 17 )2 = 16 кмч скорость лодки в стоячей воде

Автор ответа: marinagrishaeva

а почему время лодки находится как 13-1?

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