Bài 2: Tỉ số lượng giác của góc nhọn
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Hướng dẫn giải Bài 15 (Trang 77 SGK Toán Hình học 9, Tập 1)
<p><strong>B&agrave;i 15 (Trang 77 SGK To&aacute;n H&igrave;nh học 9, Tập 1):</strong></p> <p>Cho tam gi&aacute;c ABC vu&ocirc;ng tại A. Biết&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>cos</mi><mo>&#160;</mo><mover><mi mathvariant="normal">B</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>0</mn><mo>,</mo><mn>8</mn></math>. H&atilde;y t&iacute;nh c&aacute;c tỉ số lượng gi&aacute;c của g&oacute;c C.</p> <p>Gợi &yacute;: Sử dụng b&agrave;i tập 14.</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-AAcxSL.png" alt="" width="240" height="205" /></p> <p>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><mo>&#160;</mo><mo>&#8658;</mo><mo>&#160;</mo><mover><mi>B</mi><mo>^</mo></mover><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mover><mi>C</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>90</mn><mo>&#176;</mo></math></p> <p>Do vậy g&oacute;c B v&agrave; g&oacute;c C l&agrave; hai g&oacute;c phụ nhau&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo><mo>&#160;</mo><mi>sin</mi><mo>&#160;</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mi>cos</mi><mo>&#160;</mo><mover><mi mathvariant="normal">B</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>0</mn><mo>,</mo><mn>8</mn></math></p> <p>Từ kết quả b&agrave;i 14, ta c&oacute;:&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>sin</mi><mn>2</mn></msup><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><msup><mi>cos</mi><mn>2</mn></msup><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>1</mn><mo>&#160;</mo><mo>&#8660;</mo><mo>&#160;</mo><msup><mi>cos</mi><mn>2</mn></msup><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>1</mn><mo>&#160;</mo><mo>-</mo><mo>&#160;</mo><msup><mi>sin</mi><mn>2</mn></msup><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8660;</mo><mo>&#160;</mo><mi>cos</mi><mo>&#160;</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><mn>1</mn><mo>&#160;</mo><mo>-</mo><mo>&#160;</mo><msup><mi>sin</mi><mn>2</mn></msup><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover></msqrt><mo>&#160;</mo><mo>(</mo><mi>do</mi><mo>&#160;</mo><mi>g&#243;c</mi><mo>&#160;</mo><mi mathvariant="normal">C</mi><mo>&#160;</mo><mi>nh&#7885;n</mi><mo>&#160;</mo><mi>n&#234;n</mi><mo>&#160;</mo><mi>cos</mi><mo>&#160;</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>&#160;</mo><mo>&#62;</mo><mo>&#160;</mo><mn>0</mn><mo>)</mo></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8660;</mo><mo>&#160;</mo><mi>cos</mi><mo>&#160;</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><mn>1</mn><mo>&#160;</mo><mo>-</mo><mo>&#160;</mo><mn>0</mn><mo>,</mo><msup><mn>8</mn><mn>2</mn></msup></msqrt><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><mn>1</mn><mo>&#160;</mo><mo>-</mo><mo>&#160;</mo><mn>0</mn><mo>,</mo><mn>64</mn></msqrt><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><mn>0</mn><mo>,</mo><mn>36</mn></msqrt><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>0</mn><mo>,</mo><mn>6</mn></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>tan</mi><mo>&#160;</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><mi>sin</mi><mo>&#160;</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover></mrow><mrow><mi>cos</mi><mo>&#160;</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><mn>0</mn><mo>,</mo><mn>8</mn></mrow><mrow><mn>0</mn><mo>,</mo><mn>6</mn></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mn>4</mn><mn>3</mn></mfrac></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>cot</mi><mo>&#160;</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><mi>cos</mi><mo>&#160;</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover></mrow><mrow><mi>sin</mi><mo>&#160;</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><mn>0</mn><mo>,</mo><mn>6</mn></mrow><mrow><mn>0</mn><mo>,</mo><mn>8</mn></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mn>3</mn><mn>4</mn></mfrac></math></p> <p>Vậy&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>sin</mi><mo>&#160;</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>0</mn><mo>,</mo><mn>8</mn><mo>;</mo><mo>&#160;</mo><mi>cos</mi><mo>&#160;</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>0</mn><mo>,</mo><mn>6</mn><mo>;</mo><mo>&#160;</mo><mi>tan</mi><mo>&#160;</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mn>4</mn><mn>3</mn></mfrac><mo>;</mo><mo>&#160;</mo><mi>cot</mi><mo>&#160;</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mn>3</mn><mn>4</mn></mfrac><mo>.</mo></math></p>
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