Bài 2: Tỉ số lượng giác của góc nhọn
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Hướng dẫn giải Bài 11 (Trang 76 SGK Toán Hình học 9, Tập 1)
<p><strong>B&agrave;i 11 (Trang 76 SGK To&aacute;n H&igrave;nh học 9, Tập 1):</strong></p> <p>Cho tam gi&aacute;c ABC vu&ocirc;ng tại C, trong đ&oacute; AC = 0,9 m, BC = 1,2 m. T&iacute;nh c&aacute;c tỉ số lượng gi&aacute;c của g&oacute;c B, từ đ&oacute; suy ra c&aacute;c tỉ số lượng gi&aacute;c của g&oacute;c A.</p> <p>&nbsp;</p> <p><strong><span style="text-decoration: underline;"><em>Hướng dẫn giải:</em></span></strong></p> <p><strong><img class="wscnph" style="max-width: 100%;" src="https://static.colearn.vn:8413/v1.0/upload/library/10052022/bai-2-trand-12-sdk-hinh-hoc-12-1-WYPPgz.png" /></strong></p> <p><span class="mjx-mi"><span class="mjx-char MJXc-TeX-main-R">X&eacute;t tam gi&aacute;c ABC vu&ocirc;ng tại C, ta c&oacute;:</span></span></p> <p><span class="mjx-mi"><span class="mjx-char MJXc-TeX-main-R"><math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>AB</mi><mn>2</mn></msup><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msup><mi>BC</mi><mn>2</mn></msup><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><msup><mi>AC</mi><mn>2</mn></msup><mo>&#160;</mo><mo>&#8658;</mo><mo>&#160;</mo><mi>AB</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><mn>0</mn><mo>,</mo><msup><mn>9</mn><mn>2</mn></msup><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mn>1</mn><mo>,</mo><msup><mn>2</mn><mn>2</mn></msup></msqrt><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>1</mn><mo>,</mo><mn>2</mn><mo>&#160;</mo><mo>(</mo><mi mathvariant="normal">m</mi><mo>)</mo></math> (&aacute;p dụng định l&iacute; Py-ta-go)</span></span></p> <p><span class="mjx-mi"><span class="mjx-char MJXc-TeX-main-R">V&igrave; tam gi&aacute;c ABC vu&ocirc;ng tại C n&ecirc;n g&oacute;c B v&agrave; A l&agrave; hai g&oacute;c phụ nhau n&ecirc;n ta c&oacute;:</span></span></p> <p><span class="mjx-mi"><span class="mjx-char MJXc-TeX-main-R"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>sin</mi><mo>&#160;</mo><mi mathvariant="normal">A</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mi>cos</mi><mo>&#160;</mo><mi mathvariant="normal">B</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>BC</mi><mi>AB</mi></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><mn>1</mn><mo>,</mo><mn>2</mn></mrow><mrow><mn>1</mn><mo>,</mo><mn>5</mn></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mn>4</mn><mn>5</mn></mfrac></math></span></span></p> <p><span class="mjx-mi"><span class="mjx-char MJXc-TeX-main-R"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>cos</mi><mo>&#160;</mo><mi mathvariant="normal">A</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mi>sin</mi><mo>&#160;</mo><mi mathvariant="normal">B</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>AC</mi><mi>AB</mi></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><mn>0</mn><mo>,</mo><mn>9</mn></mrow><mrow><mn>1</mn><mo>,</mo><mn>5</mn></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mn>3</mn><mn>5</mn></mfrac></math></span></span></p> <p><span class="mjx-mi"><span class="mjx-char MJXc-TeX-main-R"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>tan</mi><mo>&#160;</mo><mi mathvariant="normal">A</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mi>cotg</mi><mo>&#160;</mo><mi mathvariant="normal">B</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>BC</mi><mi>AC</mi></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><mn>1</mn><mo>,</mo><mn>2</mn></mrow><mrow><mn>0</mn><mo>,</mo><mn>9</mn></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mn>4</mn><mn>3</mn></mfrac></math></span></span></p> <p><span class="mjx-mi"><span class="mjx-char MJXc-TeX-main-R"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>cotg</mi><mo>&#160;</mo><mi mathvariant="normal">A</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mi>tan</mi><mo>&#160;</mo><mi mathvariant="normal">B</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>AC</mi><mi>BC</mi></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><mn>0</mn><mo>,</mo><mn>9</mn></mrow><mrow><mn>1</mn><mo>,</mo><mn>2</mn></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mn>3</mn><mn>4</mn></mfrac></math></span></span></p>
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