Номер 408, страница 119 - гдз по алгебре 9 класс учебник Солтан, Солтан

Алгебра, 9 класс Учебник, авторы: Солтан Генадий Николаевич, Солтан Алла Евгеньевна, Жумадилова Аманбала Жумадиловна, издательство Кокшетау, Алматы, 2019

Авторы: Солтан Г. Н., Солтан А. Е., Жумадилова А. Ж.

Тип: Учебник

Издательство: Кокшетау

Год издания: 2019 - 2026

ISBN: 978-601-317-424-2

Рекомендовано Министерством образования и науки Республики Казахстан

III. Последовательности. 16. Сумма n первых членов арифметической прогрессии - номер 408, страница 119.

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            "text" => "<p><strong>408.</strong> Найдите сумму двадцати первых членов арифметической прогрессии $(a_n)$, если:</p><p><strong>а)</strong> $a_3 + a_7 + a_{14} + a_{18} = 10;$</p><p><strong>б)</strong> $a_6 + a_9 + a_{12} + a_{15} = -16.$</p>"
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            "text" => "<p><strong>408.</strong> Найдите сумму двадцати первых членов арифметической прогрессии $(a_n)$, если:</p><p><strong>а)</strong> $a_3 + a_7 + a_{14} + a_{18} = 10;$</p><p><strong>б)</strong> $a_6 + a_9 + a_{12} + a_{15} = -16.$</p>"
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            "text" => "<p><strong>а)</strong></p><p>Для решения задачи нам понадобится формула суммы первых $n$ членов арифметической прогрессии: $S_n = \frac{2a_1 + (n-1)d}{2} \cdot n$, где $a_1$ — первый член прогрессии, а $d$ — её разность.</p><p>Мы ищем сумму первых двадцати членов, то есть $S_{20}$. Подставим $n=20$ в формулу:</p><p>$S_{20} = \frac{2a_1 + (20-1)d}{2} \cdot 20 = \frac{2a_1 + 19d}{2} \cdot 20 = (2a_1 + 19d) \cdot 10$</p><p>Из условия задачи нам известно, что $a_3 + a_7 + a_{14} + a_{18} = 10$.</p><p>Воспользуемся формулой n-го члена арифметической прогрессии $a_n = a_1 + (n-1)d$, чтобы выразить каждый член суммы через $a_1$ и $d$:</p><p>$a_3 = a_1 + (3-1)d = a_1 + 2d$</p><p>$a_7 = a_1 + (7-1)d = a_1 + 6d$</p><p>$a_{14} = a_1 + (14-1)d = a_1 + 13d$</p><p>$a_{18} = a_1 + (18-1)d = a_1 + 17d$</p><p>Теперь подставим эти выражения в данное нам равенство:</p><p>$(a_1 + 2d) + (a_1 + 6d) + (a_1 + 13d) + (a_1 + 17d) = 10$</p><p>Сгруппируем и приведем подобные слагаемые:</p><p>$4a_1 + (2+6+13+17)d = 10$</p><p>$4a_1 + 38d = 10$</p><p>Разделим обе части уравнения на 2:</p><p>$2a_1 + 19d = 5$</p><p>Мы получили значение выражения $2a_1 + 19d$, которое необходимо для вычисления $S_{20}$. Подставим это значение в формулу для суммы:</p><p>$S_{20} = (2a_1 + 19d) \cdot 10 = 5 \cdot 10 = 50$</p><p><strong>Ответ:</strong> 50.</p><p><strong>б)</strong></p><p>Решение этого пункта аналогично предыдущему. Мы также ищем $S_{20}$ по формуле $S_{20} = 10(2a_1 + 19d)$.</p><p>По условию задачи дано: $a_6 + a_9 + a_{12} + a_{15} = -16$.</p><p>Выразим каждый член этой суммы через $a_1$ и $d$:</p><p>$a_6 = a_1 + 5d$</p><p>$a_9 = a_1 + 8d$</p><p>$a_{12} = a_1 + 11d$</p><p>$a_{15} = a_1 + 14d$</p><p>Подставим эти выражения в исходное равенство:</p><p>$(a_1 + 5d) + (a_1 + 8d) + (a_1 + 11d) + (a_1 + 14d) = -16$</p><p>Сгруппируем и приведем подобные слагаемые:</p><p>$4a_1 + (5+8+11+14)d = -16$</p><p>$4a_1 + 38d = -16$</p><p>Разделим обе части уравнения на 2:</p><p>$2a_1 + 19d = -8$</p><p>Теперь подставим найденное значение выражения $2a_1 + 19d$ в формулу для $S_{20}$:</p><p>$S_{20} = (2a_1 + 19d) \cdot 10 = -8 \cdot 10 = -80$</p><p><strong>Ответ:</strong> -80.</p>"
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            "text" => "<p><strong>а)</strong></p><p>Для решения задачи нам понадобится формула суммы первых $n$ членов арифметической прогрессии: $S_n = \frac{2a_1 + (n-1)d}{2} \cdot n$, где $a_1$ — первый член прогрессии, а $d$ — её разность.</p><p>Мы ищем сумму первых двадцати членов, то есть $S_{20}$. Подставим $n=20$ в формулу:</p><p>$S_{20} = \frac{2a_1 + (20-1)d}{2} \cdot 20 = \frac{2a_1 + 19d}{2} \cdot 20 = (2a_1 + 19d) \cdot 10$</p><p>Из условия задачи нам известно, что $a_3 + a_7 + a_{14} + a_{18} = 10$.</p><p>Воспользуемся формулой n-го члена арифметической прогрессии $a_n = a_1 + (n-1)d$, чтобы выразить каждый член суммы через $a_1$ и $d$:</p><p>$a_3 = a_1 + (3-1)d = a_1 + 2d$</p><p>$a_7 = a_1 + (7-1)d = a_1 + 6d$</p><p>$a_{14} = a_1 + (14-1)d = a_1 + 13d$</p><p>$a_{18} = a_1 + (18-1)d = a_1 + 17d$</p><p>Теперь подставим эти выражения в данное нам равенство:</p><p>$(a_1 + 2d) + (a_1 + 6d) + (a_1 + 13d) + (a_1 + 17d) = 10$</p><p>Сгруппируем и приведем подобные слагаемые:</p><p>$4a_1 + (2+6+13+17)d = 10$</p><p>$4a_1 + 38d = 10$</p><p>Разделим обе части уравнения на 2:</p><p>$2a_1 + 19d = 5$</p><p>Мы получили значение выражения $2a_1 + 19d$, которое необходимо для вычисления $S_{20}$. Подставим это значение в формулу для суммы:</p><p>$S_{20} = (2a_1 + 19d) \cdot 10 = 5 \cdot 10 = 50$</p><p><strong>Ответ:</strong> 50.</p><p><strong>б)</strong></p><p>Решение этого пункта аналогично предыдущему. Мы также ищем $S_{20}$ по формуле $S_{20} = 10(2a_1 + 19d)$.</p><p>По условию задачи дано: $a_6 + a_9 + a_{12} + a_{15} = -16$.</p><p>Выразим каждый член этой суммы через $a_1$ и $d$:</p><p>$a_6 = a_1 + 5d$</p><p>$a_9 = a_1 + 8d$</p><p>$a_{12} = a_1 + 11d$</p><p>$a_{15} = a_1 + 14d$</p><p>Подставим эти выражения в исходное равенство:</p><p>$(a_1 + 5d) + (a_1 + 8d) + (a_1 + 11d) + (a_1 + 14d) = -16$</p><p>Сгруппируем и приведем подобные слагаемые:</p><p>$4a_1 + (5+8+11+14)d = -16$</p><p>$4a_1 + 38d = -16$</p><p>Разделим обе части уравнения на 2:</p><p>$2a_1 + 19d = -8$</p><p>Теперь подставим найденное значение выражения $2a_1 + 19d$ в формулу для $S_{20}$:</p><p>$S_{20} = (2a_1 + 19d) \cdot 10 = -8 \cdot 10 = -80$</p><p><strong>Ответ:</strong> -80.</p>"
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№408 (с. 119)
Условие. №408 (с. 119)
скриншот условия
Алгебра, 9 класс Учебник, авторы: Солтан Генадий Николаевич, Солтан Алла Евгеньевна, Жумадилова Аманбала Жумадиловна, издательство Кокшетау, Алматы, 2019, страница 119, номер 408, Условие

408. Найдите сумму двадцати первых членов арифметической прогрессии $(a_n)$, если:

а) $a_3 + a_7 + a_{14} + a_{18} = 10;$

б) $a_6 + a_9 + a_{12} + a_{15} = -16.$

Решение. №408 (с. 119)
Алгебра, 9 класс Учебник, авторы: Солтан Генадий Николаевич, Солтан Алла Евгеньевна, Жумадилова Аманбала Жумадиловна, издательство Кокшетау, Алматы, 2019, страница 119, номер 408, Решение
Решение 2 (rus). №408 (с. 119)

а)

Для решения задачи нам понадобится формула суммы первых $n$ членов арифметической прогрессии: $S_n = \frac{2a_1 + (n-1)d}{2} \cdot n$, где $a_1$ — первый член прогрессии, а $d$ — её разность.

Мы ищем сумму первых двадцати членов, то есть $S_{20}$. Подставим $n=20$ в формулу:

$S_{20} = \frac{2a_1 + (20-1)d}{2} \cdot 20 = \frac{2a_1 + 19d}{2} \cdot 20 = (2a_1 + 19d) \cdot 10$

Из условия задачи нам известно, что $a_3 + a_7 + a_{14} + a_{18} = 10$.

Воспользуемся формулой n-го члена арифметической прогрессии $a_n = a_1 + (n-1)d$, чтобы выразить каждый член суммы через $a_1$ и $d$:

$a_3 = a_1 + (3-1)d = a_1 + 2d$

$a_7 = a_1 + (7-1)d = a_1 + 6d$

$a_{14} = a_1 + (14-1)d = a_1 + 13d$

$a_{18} = a_1 + (18-1)d = a_1 + 17d$

Теперь подставим эти выражения в данное нам равенство:

$(a_1 + 2d) + (a_1 + 6d) + (a_1 + 13d) + (a_1 + 17d) = 10$

Сгруппируем и приведем подобные слагаемые:

$4a_1 + (2+6+13+17)d = 10$

$4a_1 + 38d = 10$

Разделим обе части уравнения на 2:

$2a_1 + 19d = 5$

Мы получили значение выражения $2a_1 + 19d$, которое необходимо для вычисления $S_{20}$. Подставим это значение в формулу для суммы:

$S_{20} = (2a_1 + 19d) \cdot 10 = 5 \cdot 10 = 50$

Ответ: 50.

б)

Решение этого пункта аналогично предыдущему. Мы также ищем $S_{20}$ по формуле $S_{20} = 10(2a_1 + 19d)$.

По условию задачи дано: $a_6 + a_9 + a_{12} + a_{15} = -16$.

Выразим каждый член этой суммы через $a_1$ и $d$:

$a_6 = a_1 + 5d$

$a_9 = a_1 + 8d$

$a_{12} = a_1 + 11d$

$a_{15} = a_1 + 14d$

Подставим эти выражения в исходное равенство:

$(a_1 + 5d) + (a_1 + 8d) + (a_1 + 11d) + (a_1 + 14d) = -16$

Сгруппируем и приведем подобные слагаемые:

$4a_1 + (5+8+11+14)d = -16$

$4a_1 + 38d = -16$

Разделим обе части уравнения на 2:

$2a_1 + 19d = -8$

Теперь подставим найденное значение выражения $2a_1 + 19d$ в формулу для $S_{20}$:

$S_{20} = (2a_1 + 19d) \cdot 10 = -8 \cdot 10 = -80$

Ответ: -80.

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