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 13 (Trang 77 SGK Toán Hình học 9, Tập 1)
<p><strong>B&agrave;i 13 (Trang 77 SGK To&aacute;n H&igrave;nh học 9, Tập 1):</strong></p> <p>Dựng g&oacute;c nhọn &alpha;, biết:</p> <p>a) sin &alpha; = <math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mn>2</mn><mn>3</mn></mfrac></math>;</p> <p>b) cos &alpha; = <math xmlns="http://www.w3.org/1998/Math/MathML"><mn>0</mn><mo>,</mo><mn>6</mn></math>;</p> <p>c) tan &alpha; = <math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mn>3</mn><mn>4</mn></mfrac></math>;</p> <p>d) cot &alpha; = <math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mn>3</mn><mn>2</mn></mfrac></math>;</p> <p>&nbsp;</p> <p><strong><span style="text-decoration: underline;"><em>Hướng dẫn giải:</em></span></strong></p> <p>a)</p> <p>Ta dựng như sau:</p> <p>- Vẽ g&oacute;c vu&ocirc;ng&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>xOy</mi><mo>^</mo></mover></math></p> <p>- Tr&ecirc;n tia Ox lấy điểm A sao cho OA = 2 đơn vị</p> <p>- Lấy A l&agrave;m t&acirc;m, vẽ cung tr&ograve;n b&aacute;n k&iacute;nh 3 đơn vị sao cho cung tr&ograve;n cắt tia Oy tại B.</p> <p>- Khi đ&oacute;, ta lấy&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>OBA</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi></math> l&agrave; g&oacute;c cần dựng.</p> <p>Ta c&oacute;&nbsp;<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><mover><mi>OBA</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>OA</mi><mi>AB</mi></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mn>2</mn><mn>3</mn></mfrac></math></p> <p><img src="https://static.colearn.vn:8413/v1.0/upload/library/10052022/bai-13-trand-77-sdk-toan-9-tap-1-5-sua2022-Qq3UMg.png" alt="" width="252" height="257" /></p> <p>&nbsp;</p> <p><strong>b)</strong></p> <p><strong>Ta dựng như sau:</strong></p> <p>- Vẽ g&oacute;c vu&ocirc;ng <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>xOy</mi><mo>^</mo></mover></math></p> <p>- Tr&ecirc;n tia Ox lấy điểm A sao cho OA = 3 đơn vị</p> <p>- Lấy A l&agrave;m t&acirc;m, vẽ cung tr&ograve;n b&aacute;n k&iacute;nh 5 đơn vị sao cho cung tr&ograve;n cắt tia Oy tại B</p> <p>- Khi đ&oacute; ta c&oacute;&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>OAB</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi></math> l&agrave; g&oacute;c cần dựng</p> <p>Ta c&oacute;&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><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><mover><mi>OAB</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>OA</mi><mi>AB</mi></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mn>3</mn><mn>5</mn></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>0</mn><mo>,</mo><mn>6</mn></math></p> <p><img src="https://static.colearn.vn:8413/v1.0/upload/library/10052022/bai-13-trand-77-sdk-toan-9-tap-1-5-sua2022-DSOSIf.png" alt="" width="275" height="275" /></p> <p><strong>c)</strong></p> <p>Ta dựng như sau:</p> <p>- Vẽ g&oacute;c vu&ocirc;ng&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>xOy</mi><mo>^</mo></mover></math></p> <p>- Tr&ecirc;n tia Ox lấy điểm A sao cho OA = 3 đơn vị.</p> <p>- Tr&ecirc;n tia Oy lấy điểm B sao cho OB = 4 đơn vị.</p> <p>- Khi đ&oacute;&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>OBA</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi></math> l&agrave; g&oacute;c cần dựng.</p> <p>Ta c&oacute;&nbsp;<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>tan</mi><mo>&#160;</mo><mover><mi>OBA</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>OA</mi><mi>OB</mi></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mn>3</mn><mn>4</mn></mfrac></math></p> <p>- Vẽ g&oacute;c vu&ocirc;ng&nbsp;<span id="MathJax-Element-10-Frame" class="mjx-chtml MathJax_CHTML" style="box-sizing: border-box; display: inline-block; line-height: 0; text-indent: 0px; text-align: left; text-transform: none; font-style: normal; font-weight: normal; font-size: 21.78px; letter-spacing: normal; overflow-wrap: normal; word-spacing: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; margin: 0px; padding: 1px 0px; position: relative;" tabindex="0" role="presentation" data-mathml="&lt;math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;&gt;&lt;mover accent=&quot;true&quot;&gt;&lt;mrow&gt;&lt;mi&gt;x&lt;/mi&gt;&lt;mi&gt;O&lt;/mi&gt;&lt;mi&gt;y&lt;/mi&gt;&lt;/mrow&gt;&lt;mo&gt;^&lt;/mo&gt;&lt;/mover&gt;&lt;/math&gt;"><span id="MJXc-Node-96" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-97" class="mjx-mrow"><span id="MJXc-Node-98" class="mjx-mover"><span class="mjx-stack"><span class="mjx-over"><span id="MJXc-Node-103" class="mjx-mo"></span></span></span></span></span></span></span><math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mrow><mi>x</mi><mi>O</mi><mi>y</mi></mrow><mo>^</mo></mover></math></p> <p>- Tr&ecirc;n tia Ox lấy điểm A sao cho OA = 3 đơn vị</p> <p>- Tr&ecirc;n tia Oy lấy điểm B sao cho OB = 4 đơn vị</p> <p><img src="https://static.colearn.vn:8413/v1.0/upload/library/10052022/bai-13-trand-77-sdk-toan-9-tap-1-5-sua2022-naqrQb.png" alt="" width="250" height="258" /></p> <p><strong>d)</strong></p> <p>Ta dựng như sau:</p> <p>- Vẽ g&oacute;c vu&ocirc;ng&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>xOy</mi><mo>^</mo></mover></math></p> <p>- Tr&ecirc;n tia Ox lấy điểm A sao cho OA = 2 đơn vị</p> <p>- Tr&ecirc;n tia Oy lấy điểm B sao cho OB = 3 đơn vị</p> <p>- Khi đ&oacute;&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>OBA</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mi mathvariant="normal">&#945;</mi></math> l&agrave; g&oacute;c cần dựng.</p> <p>Ta c&oacute;&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><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><mover><mi>OBA</mi><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>OB</mi><mi>OA</mi></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mn>3</mn><mn>2</mn></mfrac></math></p> <p><img src="https://static.colearn.vn:8413/v1.0/upload/library/10052022/bai-13-trand-77-sdk-toan-9-tap-1-5-sua2022-A1XoYm.png" alt="" width="260" height="256" /></p>
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