Bài 2: Tỉ số lượng giác của góc nhọn
<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>&#945;</mi><mo>&#160;</mo><mi>v</mi><mi>&#224;</mi><mo>&#160;</mo><mi>&#946;</mi></math>
Hướng dẫn giải Bài 14 (Trang 77 SGK Toán Hình học 9, Tập 1)
<p><strong>B&agrave;i 14 (Trang 77 SGK To&aacute;n H&igrave;nh học 9, Tập 1):</strong></p> <p>Sử dụng định nghĩa c&aacute;c tỉ số lượng gi&aacute;c của một g&oacute;c nhọn để chứng minh rằng: Với g&oacute;c nhọn &alpha; t&ugrave;y &yacute;, ta c&oacute;:&nbsp;</p> <p>a)&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>tg</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><mi>sin</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi></mrow><mrow><mi>cos</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi></mrow></mfrac><mo>;</mo><mo>&#160;</mo><mi>cotg</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><mi>cos</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi></mrow><mrow><mi>sin</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi></mrow></mfrac><mo>;</mo><mo>&#160;</mo><mi>tg</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>.</mo><mo>&#160;</mo><mi>cotg</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>1</mn><mo>;</mo></math></p> <p>b)&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>sin</mi><mn>2</mn></msup><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><msup><mi>cos</mi><mn>2</mn></msup><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>1</mn></math></p> <p>Gợi &yacute;: Sử dụng định l&iacute; Py-ta-go.</p> <p>&nbsp;</p> <p><strong><span style="text-decoration: underline;"><em>Hướng dẫn giải:</em></span></strong></p> <p><img src="https://static.colearn.vn:8413/v1.0/upload/library/10052022/bai-14-trand-77-sdk-toan-9-tap-1-3-sua2022-eoFh6Y.png" alt="" width="240" height="205" /></p> <p style="text-align: left;">a) X&eacute;t tam gi&aacute;c ABC vu&ocirc;ng tại A c&oacute;&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi mathvariant="normal">A</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>90</mn><mo>&#176;</mo></math></p> <p style="text-align: left;"><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo><mo>&#160;</mo><mover><mi mathvariant="normal">B</mi><mo>^</mo></mover><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>90</mn><mo>&#176;</mo></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo><mo>&#160;</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>&#160;</mo><mo>&#60;</mo><mo>&#160;</mo><mn>90</mn><mo>&#176;</mo></math></p> <p>Đặt&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mi>hay</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>&#60;</mo><mo>&#160;</mo><mn>90</mn><mo>&#176;</mo></math></p> <p>Do đ&oacute;&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>&#945;</mi></math> l&agrave; g&oacute;c nhọn.</p> <p>C&aacute;c tỉ số lượng gi&aacute;c của g&oacute;c&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi></math> như sau:</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>sin</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mi>sin</mi><mo>&#160;</mo><mi mathvariant="normal">C</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>AB</mi><mi>BC</mi></mfrac><mspace linebreak="newline"/><mi>cos</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mi>cos</mi><mo>&#160;</mo><mi mathvariant="normal">C</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>AC</mi><mi>BC</mi></mfrac><mspace linebreak="newline"/><mi>tan</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mi>tan</mi><mo>&#160;</mo><mi mathvariant="normal">C</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>AC</mi><mi>AB</mi></mfrac><mspace linebreak="newline"/><mi>cot</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mi>cot</mi><mo>&#160;</mo><mi mathvariant="normal">C</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>AC</mi><mi>AB</mi></mfrac></math></p> <p>Ta c&oacute; thể thấy rằng:</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mi>sin</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi></mrow><mrow><mi>cos</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>AB</mi><mi>BC</mi></mfrac><mo>:</mo><mfrac><mi>AC</mi><mi>BC</mi></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>AB</mi><mi>BC</mi></mfrac><mo>.</mo><mfrac><mi>BC</mi><mi>AC</mi></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mi>tan</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>&#160;</mo><mo>(</mo><mi>&#273;pcm</mi><mo>)</mo></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mi>cos</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi></mrow><mrow><mi>sin</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>AC</mi><mi>BC</mi></mfrac><mo>:</mo><mfrac><mi>AB</mi><mi>BC</mi></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>AC</mi><mi>BC</mi></mfrac><mo>.</mo><mfrac><mi>BC</mi><mi>AB</mi></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>AC</mi><mi>AB</mi></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mi>cot</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>(</mo><mi>&#273;pcm</mi><mo>)</mo></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>tan</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>.</mo><mo>&#160;</mo><mi>cot</mi><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>AB</mi><mi>AC</mi></mfrac><mo>.</mo><mfrac><mi>AC</mi><mi>AB</mi></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>1</mn><mo>&#160;</mo><mo>(</mo><mi>&#273;pcm</mi><mo>)</mo></math></p> <p>b) X&eacute;t tam gi&aacute;c ABC vu&ocirc;ng tại A, ta c&oacute;:</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>BC</mi><mn>2</mn></msup><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msup><mi>AB</mi><mn>2</mn></msup><mo>&#160;</mo><mo>+</mo><mo>&#8201;</mo><msup><mi>AC</mi><mn>2</mn></msup><mo>&#160;</mo><mo>(</mo><mi>theo</mi><mo>&#160;</mo><mi>&#273;&#7883;nh</mi><mo>&#160;</mo><mi>l&#237;</mi><mo>&#160;</mo><mi>Py</mi><mo>-</mo><mi>ta</mi><mo>-</mo><mi>go</mi><mo>)</mo><mo>&#160;</mo><mo>(</mo><mn>1</mn><mo>)</mo></math></p> <p>Mặt kh&aacute;c, ta c&oacute;:</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>sin</mi><mn>2</mn></msup><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msup><mfenced><mfrac><mi>AB</mi><mi>BC</mi></mfrac></mfenced><mn>2</mn></msup><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><msup><mi>AB</mi><mn>2</mn></msup><msup><mi>BC</mi><mn>2</mn></msup></mfrac></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>cos</mi><mn>2</mn></msup><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msup><mfenced><mfrac><mi>AB</mi><mi>BC</mi></mfrac></mfenced><mn>2</mn></msup><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><msup><mi>AB</mi><mn>2</mn></msup><msup><mi>BC</mi><mn>2</mn></msup></mfrac></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo><mo>&#160;</mo><msup><mi>sin</mi><mn>2</mn></msup><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><msup><mi>cos</mi><mn>2</mn></msup><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><msup><mi>AB</mi><mn>2</mn></msup><msup><mi>BC</mi><mn>2</mn></msup></mfrac><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mfrac><msup><mi>AC</mi><mn>2</mn></msup><msup><mi>BC</mi><mn>2</mn></msup></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><msup><mi>AB</mi><mn>2</mn></msup><mo>&#160;</mo><mo>+</mo><mo>&#8201;</mo><msup><mi>AC</mi><mn>2</mn></msup></mrow><msup><mi>BC</mi><mn>2</mn></msup></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><msup><mi>BC</mi><mn>2</mn></msup><msup><mi>BC</mi><mn>2</mn></msup></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>1</mn><mo>&#160;</mo><mo>(</mo><mi>theo</mi><mo>&#160;</mo><mo>(</mo><mn>1</mn><mo>)</mo><mo>)</mo></math></p> <p>Vậy&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>sin</mi><mn>2</mn></msup><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><msup><mi>cos</mi><mn>2</mn></msup><mi mathvariant="normal">&#945;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>1</mn><mo>&#160;</mo><mo>(</mo><mi>&#273;pcm</mi><mo>)</mo></math></p>
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