14(C2). Расстояние от вершины А до сечения в правильной четырёхугольной пирамиде (вар. 58)
В правильной четырехугольной пирамиде PABCD с основанием ABCD точка M - середина ребра РA, точка K - середина ребра РB. Найдите расстояние от вершины A до плоскости CMK, если РC = 6, AB = 4.
Сделать чертёж и построить сечение (трапецию) в данной задаче несложно.
МК - средняя линия треугольника АРВ, следовательно, МК || АВ. А это означает, что АВ параллельна плоскости сечения по признаку параллельности прямой и плоскости.
Расстояние от точки А до сечения равно расстоянию от прямой АВ до сечения, а оно, в свою очередь, равно расстоянию от абсолютно любой точки прямой АВ до сечения.-----------------------------------------------
В качестве такой точки удобно рассмотреть середину отрезка АВ - точку Н (АН = НВ).
Вся наша конструкция симметрична относительно плоскости НРТ, Т - середина DC.
Плоскость симметрии перпендикулярна сечению * и пересекаются они по прямой РТ.
По свойству перпендикулярных плоскостей перпендикуляр, опущенный из т. Н на сечение,
попадает точно на прямую РТ, то есть найти нам надо длину именно этого перпендикуляра.
Итак, задача сводится к нахождению высоты треугольника НРТ, проведённой к стороне РТ.
Ну а сделать это очень просто, найдя площадь треугольника двумя разными способами:
Чтобы найти искомое расстояние, обозначенное через х, надо найти PT, HT и PL.1) PL = 0,5·РО, т.к. PL - ср. линия треугольника НРО. Найдём высоту пирамиды.В прямоугольном треугольнике PОС: PС = 6 и ОС = 0,5·АС = 0,5·4√2 = 2√2.По теореме Пифагора находим РО = √36 - 8 = √28 = 2√7. Значит, PL = √7.-----------------------------------------------2) НТ = ВС = АВ = 4. HL = 0,5·HO = 0,5·2 = 1, LT = HT - HL = 4 - 1 = 3.-----------------------------------------------3) PT найдём по теореме Пифагора из треугольника PLT: PT = √9 + 7 = 4.================================================
Теперь можно искать высоту х, проведённую к стороне РТ треугольника PTL:
HT·PL = PT·x4·√7 = 4·xx = √7Ответ: √7* Докажем, что плоскость симметрии перпендикулярна плоскости сечения.Плоскость сечения проходит через прямую DC, которая перпендикулярна плоскости симметрии НРТ. (Но почему? Докажите это самостоятельно.)По признаку перпендикулярности плоскостей плоскости перпендикулярны. Автор: Ольга Себедаш Просмотров: 21036
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Тр-кHTP оказался равнобедренным. Высоты на боковые стороны равны.Т.е.,искомое расстояние равно половине высоты пирамиды.p.s.У Вас на рис.2точки P.
Раиса, я и не находила боковую сторону, поэтому равнобедренность не видела. Впрочем, это частный случай. Хорошо уметь находить высоту треугольника в общем случае. Да, с точкой Р я увлеклась. Исправлю со временем, спасибо!
У вас на рисунке плоскость CMKD, а по условию - CMK
Елена, три точки задают плоскость. Четвёртая точка на этой плоскости лежит. Построено сечение.
решала через метод координат ответ не сходится
Эльвира, что я могу... Ищите ошибку.
Я не понял почему на рисунке две точки P
где будет расстояние от вершины пирамиды - точки Р до секущей плоскости?
В правильной четырёхугольной пирамиде MABCD с вершиной M стороны основания равны 3, а боковые рёбра равны 8.
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