Номер 726, страница 207 - 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

IV. Тригонометрия. 27. Формулы тригонометрических функций двойного и половинного углов - номер 726, страница 207.

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            "text" => "<p><strong>726.</strong> Учащимся было дано задание: найти при $\alpha = 120^\circ$ значение выражения $\sqrt{(1 - \cos \alpha)^2 + \sin^2 \alpha}$.</p><p><strong>Николай выполнил задание так:</strong></p><p>$\sqrt{(1 - \cos \alpha)^2 + \sin^2 \alpha} = \sqrt{(1 - \cos 60^\circ)^2 + \sin^2 60^\circ} = \sqrt{\frac{1}{4} + \frac{3}{4}} = 1$.</p><p><strong>Василий решал так:</strong></p><p>$\sqrt{(1 - \cos \alpha)^2 + \sin^2 \alpha} = \sqrt{1 - 2 \cos \alpha + \cos^2 \alpha + \sin^2 \alpha} = \sqrt{2(1 - \cos \alpha)} = \sqrt{4 \sin^2 \frac{\alpha}{2}} = 2 \left| \sin \frac{\alpha}{2} \right| = 2 \sin 60^\circ = \sqrt{3}$.</p><p>Чье решение было правильным?</p>"
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            "text" => "<p>Чтобы определить, чьё решение является правильным, необходимо проанализировать шаги каждого ученика и сравнить их с верным решением. Задание состоит в том, чтобы найти значение выражения $\sqrt{(1 - \cos \alpha)^2 + \sin^2 \alpha}$ при $\alpha = 120^\circ$.</p><p>Сначала рассмотрим решение <strong>Николая</strong>. Он начал с подстановки $\sqrt{(1 - \cos 60^\circ)^2 + \sin^2 60^\circ}$. Это является ошибкой, так как по условию задачи $\alpha = 120^\circ$, а не $60^\circ$. Хотя его дальнейшие вычисления для угла $60^\circ$ верны ($\sqrt{(1 - \frac{1}{2})^2 + (\frac{\sqrt{3}}{2})^2} = \sqrt{(\frac{1}{2})^2 + \frac{3}{4}} = \sqrt{\frac{1}{4} + \frac{3}{4}} = \sqrt{1} = 1$), результат не является решением исходной задачи из-за неверной подстановки.</p><p>Теперь проанализируем решение <strong>Василия</strong>. Он выбрал путь упрощения выражения перед подстановкой значения угла.<br>1. Он раскрывает скобки под корнем: $\sqrt{1 - 2\cos \alpha + \cos^2 \alpha + \sin^2 \alpha}$.<br>2. Затем использует основное тригонометрическое тождество $\sin^2 \alpha + \cos^2 \alpha = 1$, что приводит выражение к виду $\sqrt{1 - 2\cos \alpha + 1} = \sqrt{2 - 2\cos \alpha}$.<br>3. Далее он выносит общий множитель и применяет формулу половинного угла $1 - \cos \alpha = 2\sin^2 \frac{\alpha}{2}$: $\sqrt{2(1 - \cos \alpha)} = \sqrt{2 \cdot 2\sin^2 \frac{\alpha}{2}} = \sqrt{4\sin^2 \frac{\alpha}{2}}$.<br>4. Наконец, он правильно извлекает корень: $\sqrt{4\sin^2 \frac{\alpha}{2}} = |2\sin \frac{\alpha}{2}| = 2|\sin \frac{\alpha}{2}|$.<br>Все преобразования выполнены верно.<br>После этого Василий подставляет $\alpha = 120^\circ$:<br>$2|\sin \frac{120^\circ}{2}| = 2|\sin 60^\circ|$.<br>Так как $\sin 60^\circ &gt; 0$, модуль можно убрать: $2\sin 60^\circ = 2 \cdot \frac{\sqrt{3}}{2} = \sqrt{3}$.<br>Все шаги Василия логичны и математически корректны.</p><p>Для окончательной проверки можно подставить значение $\alpha = 120^\circ$ напрямую в исходное выражение:<br>$\cos 120^\circ = -\frac{1}{2}$<br>$\sin 120^\circ = \frac{\sqrt{3}}{2}$<br>$\sqrt{(1 - (-\frac{1}{2}))^2 + (\frac{\sqrt{3}}{2})^2} = \sqrt{(1 + \frac{1}{2})^2 + \frac{3}{4}} = \sqrt{(\frac{3}{2})^2 + \frac{3}{4}} = \sqrt{\frac{9}{4} + \frac{3}{4}} = \sqrt{\frac{12}{4}} = \sqrt{3}$.<br>Этот результат совпадает с ответом Василия.</p><p><strong>Ответ:</strong> Правильным было решение Василия.</p>"
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            "text" => "<p>Чтобы определить, чьё решение является правильным, необходимо проанализировать шаги каждого ученика и сравнить их с верным решением. Задание состоит в том, чтобы найти значение выражения $\sqrt{(1 - \cos \alpha)^2 + \sin^2 \alpha}$ при $\alpha = 120^\circ$.</p><p>Сначала рассмотрим решение <strong>Николая</strong>. Он начал с подстановки $\sqrt{(1 - \cos 60^\circ)^2 + \sin^2 60^\circ}$. Это является ошибкой, так как по условию задачи $\alpha = 120^\circ$, а не $60^\circ$. Хотя его дальнейшие вычисления для угла $60^\circ$ верны ($\sqrt{(1 - \frac{1}{2})^2 + (\frac{\sqrt{3}}{2})^2} = \sqrt{(\frac{1}{2})^2 + \frac{3}{4}} = \sqrt{\frac{1}{4} + \frac{3}{4}} = \sqrt{1} = 1$), результат не является решением исходной задачи из-за неверной подстановки.</p><p>Теперь проанализируем решение <strong>Василия</strong>. Он выбрал путь упрощения выражения перед подстановкой значения угла.<br>1. Он раскрывает скобки под корнем: $\sqrt{1 - 2\cos \alpha + \cos^2 \alpha + \sin^2 \alpha}$.<br>2. Затем использует основное тригонометрическое тождество $\sin^2 \alpha + \cos^2 \alpha = 1$, что приводит выражение к виду $\sqrt{1 - 2\cos \alpha + 1} = \sqrt{2 - 2\cos \alpha}$.<br>3. Далее он выносит общий множитель и применяет формулу половинного угла $1 - \cos \alpha = 2\sin^2 \frac{\alpha}{2}$: $\sqrt{2(1 - \cos \alpha)} = \sqrt{2 \cdot 2\sin^2 \frac{\alpha}{2}} = \sqrt{4\sin^2 \frac{\alpha}{2}}$.<br>4. Наконец, он правильно извлекает корень: $\sqrt{4\sin^2 \frac{\alpha}{2}} = |2\sin \frac{\alpha}{2}| = 2|\sin \frac{\alpha}{2}|$.<br>Все преобразования выполнены верно.<br>После этого Василий подставляет $\alpha = 120^\circ$:<br>$2|\sin \frac{120^\circ}{2}| = 2|\sin 60^\circ|$.<br>Так как $\sin 60^\circ &gt; 0$, модуль можно убрать: $2\sin 60^\circ = 2 \cdot \frac{\sqrt{3}}{2} = \sqrt{3}$.<br>Все шаги Василия логичны и математически корректны.</p><p>Для окончательной проверки можно подставить значение $\alpha = 120^\circ$ напрямую в исходное выражение:<br>$\cos 120^\circ = -\frac{1}{2}$<br>$\sin 120^\circ = \frac{\sqrt{3}}{2}$<br>$\sqrt{(1 - (-\frac{1}{2}))^2 + (\frac{\sqrt{3}}{2})^2} = \sqrt{(1 + \frac{1}{2})^2 + \frac{3}{4}} = \sqrt{(\frac{3}{2})^2 + \frac{3}{4}} = \sqrt{\frac{9}{4} + \frac{3}{4}} = \sqrt{\frac{12}{4}} = \sqrt{3}$.<br>Этот результат совпадает с ответом Василия.</p><p><strong>Ответ:</strong> Правильным было решение Василия.</p>"
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№726 (с. 207)
Условие. №726 (с. 207)
скриншот условия
Алгебра, 9 класс Учебник, авторы: Солтан Генадий Николаевич, Солтан Алла Евгеньевна, Жумадилова Аманбала Жумадиловна, издательство Кокшетау, Алматы, 2019, страница 207, номер 726, Условие

726. Учащимся было дано задание: найти при $\alpha = 120^\circ$ значение выражения $\sqrt{(1 - \cos \alpha)^2 + \sin^2 \alpha}$.

Николай выполнил задание так:

$\sqrt{(1 - \cos \alpha)^2 + \sin^2 \alpha} = \sqrt{(1 - \cos 60^\circ)^2 + \sin^2 60^\circ} = \sqrt{\frac{1}{4} + \frac{3}{4}} = 1$.

Василий решал так:

$\sqrt{(1 - \cos \alpha)^2 + \sin^2 \alpha} = \sqrt{1 - 2 \cos \alpha + \cos^2 \alpha + \sin^2 \alpha} = \sqrt{2(1 - \cos \alpha)} = \sqrt{4 \sin^2 \frac{\alpha}{2}} = 2 \left| \sin \frac{\alpha}{2} \right| = 2 \sin 60^\circ = \sqrt{3}$.

Чье решение было правильным?

Решение. №726 (с. 207)
Алгебра, 9 класс Учебник, авторы: Солтан Генадий Николаевич, Солтан Алла Евгеньевна, Жумадилова Аманбала Жумадиловна, издательство Кокшетау, Алматы, 2019, страница 207, номер 726, Решение
Решение 2 (rus). №726 (с. 207)

Чтобы определить, чьё решение является правильным, необходимо проанализировать шаги каждого ученика и сравнить их с верным решением. Задание состоит в том, чтобы найти значение выражения $\sqrt{(1 - \cos \alpha)^2 + \sin^2 \alpha}$ при $\alpha = 120^\circ$.

Сначала рассмотрим решение Николая. Он начал с подстановки $\sqrt{(1 - \cos 60^\circ)^2 + \sin^2 60^\circ}$. Это является ошибкой, так как по условию задачи $\alpha = 120^\circ$, а не $60^\circ$. Хотя его дальнейшие вычисления для угла $60^\circ$ верны ($\sqrt{(1 - \frac{1}{2})^2 + (\frac{\sqrt{3}}{2})^2} = \sqrt{(\frac{1}{2})^2 + \frac{3}{4}} = \sqrt{\frac{1}{4} + \frac{3}{4}} = \sqrt{1} = 1$), результат не является решением исходной задачи из-за неверной подстановки.

Теперь проанализируем решение Василия. Он выбрал путь упрощения выражения перед подстановкой значения угла.
1. Он раскрывает скобки под корнем: $\sqrt{1 - 2\cos \alpha + \cos^2 \alpha + \sin^2 \alpha}$.
2. Затем использует основное тригонометрическое тождество $\sin^2 \alpha + \cos^2 \alpha = 1$, что приводит выражение к виду $\sqrt{1 - 2\cos \alpha + 1} = \sqrt{2 - 2\cos \alpha}$.
3. Далее он выносит общий множитель и применяет формулу половинного угла $1 - \cos \alpha = 2\sin^2 \frac{\alpha}{2}$: $\sqrt{2(1 - \cos \alpha)} = \sqrt{2 \cdot 2\sin^2 \frac{\alpha}{2}} = \sqrt{4\sin^2 \frac{\alpha}{2}}$.
4. Наконец, он правильно извлекает корень: $\sqrt{4\sin^2 \frac{\alpha}{2}} = |2\sin \frac{\alpha}{2}| = 2|\sin \frac{\alpha}{2}|$.
Все преобразования выполнены верно.
После этого Василий подставляет $\alpha = 120^\circ$:
$2|\sin \frac{120^\circ}{2}| = 2|\sin 60^\circ|$.
Так как $\sin 60^\circ > 0$, модуль можно убрать: $2\sin 60^\circ = 2 \cdot \frac{\sqrt{3}}{2} = \sqrt{3}$.
Все шаги Василия логичны и математически корректны.

Для окончательной проверки можно подставить значение $\alpha = 120^\circ$ напрямую в исходное выражение:
$\cos 120^\circ = -\frac{1}{2}$
$\sin 120^\circ = \frac{\sqrt{3}}{2}$
$\sqrt{(1 - (-\frac{1}{2}))^2 + (\frac{\sqrt{3}}{2})^2} = \sqrt{(1 + \frac{1}{2})^2 + \frac{3}{4}} = \sqrt{(\frac{3}{2})^2 + \frac{3}{4}} = \sqrt{\frac{9}{4} + \frac{3}{4}} = \sqrt{\frac{12}{4}} = \sqrt{3}$.
Этот результат совпадает с ответом Василия.

Ответ: Правильным было решение Василия.

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