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Hướng dẫn giải Bài 9 (Trang 111 SGK Toán 7, Bộ Kết nối tri thức, Tập 2)
<p><strong>B&agrave;i 9 (Trang 111 SGK To&aacute;n 7, Bộ Kết nối tri thức với cuộc sống, Tập 2)</strong></p> <p>Cho tam gi&aacute;c c&acirc;n ABC tại đỉnh A. Gọi H l&agrave; trung điểm của BC.</p> <p>a) Chứng minh AH &perp; BC.</p> <p>b) Tr&ecirc;n tia đối của tia BC lấy điểm M; tr&ecirc;n tia đối của tia CB lấy điểm N sao cho BM = CN. Chứng minh rằng <math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8710;</mo></math>ABM = <math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8710;</mo></math>ACN.</p> <p>c) Gọi I l&agrave; điểm tr&ecirc;n AM, K l&agrave; điểm tr&ecirc;n AN sao cho BI <math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8869;</mo></math> AM; CK &perp; AN. Chứng minh rằng tam gi&aacute;c AIK c&acirc;n tại A, từ đ&oacute; suy ra IK // MN.</p> <p>&nbsp;</p> <p><em><span style="text-decoration: underline;"><strong>Hướng dẫn giải</strong></span></em></p> <p><img class="wscnph" style="max-width: 100%; display: block; margin-left: auto; margin-right: auto;" src="https://static.colearn.vn:8413/v1.0/upload/library/07102022/bai-9-trand-111-toan-lop-7-tap-2-148061-ke4qQP.png" width="426" height="248" /></p> <p>a) Do H l&agrave; trung điểm của BC n&ecirc;n BH = CH.</p> <p>Tam gi&aacute;c ABC c&acirc;n tại A n&ecirc;n AB = AC v&agrave;&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mrow><mi>A</mi><mi>B</mi><mi>C</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>C</mi><mi>B</mi></mrow><mo>^</mo></mover></math></p> <p>X&eacute;t <math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8710;</mo></math>ABH v&agrave; <math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8710;</mo></math>ACH c&oacute;:</p> <p>AB = AC (chứng minh tr&ecirc;n).</p> <p>BH chung.</p> <p>BH = CH (chứng minh tr&ecirc;n).</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo></math> ∆ABH = ∆ACH (c - c - c).</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>H</mi><mi>B</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>H</mi><mi>C</mi></mrow><mo>^</mo></mover><mo>&#160;</mo></math>(2 g&oacute;c tương ứng)</p> <p>m&agrave;&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mrow><mi>A</mi><mi>H</mi><mi>B</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>H</mi><mi>C</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>180</mn><mo>&#176;</mo><mo>&#160;</mo><mo>&#8658;</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>H</mi><mi>B</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>H</mi><mi>C</mi></mrow><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><mi>A</mi><mi>H</mi><mo>&#160;</mo><mo>&#8869;</mo><mo>&#160;</mo><mi>B</mi><mi>C</mi><mo>.</mo></math></p> <p>&nbsp;</p> <p>b) Ta c&oacute;:&nbsp;</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mrow><mi>A</mi><mi>B</mi><mi>M</mi></mrow><mo>^</mo></mover></math> l&agrave; g&oacute;c ngo&agrave;i tại đỉnh B của&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8710;</mo><mi>A</mi><mi>B</mi><mi>C</mi></math>&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>B</mi><mi>M</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mover><mrow><mi>B</mi><mi>A</mi><mi>C</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>C</mi><mi>B</mi></mrow><mo>^</mo></mover></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mrow><mi>A</mi><mi>C</mi><mi>N</mi></mrow><mo>^</mo></mover></math> l&agrave; g&oacute;c ngo&agrave;i tại đỉnh c của&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8710;</mo><mi>A</mi><mi>B</mi><mi>C</mi><mo>&#160;</mo><mo>&#8658;</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>C</mi><mi>N</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mover><mrow><mi>B</mi><mi>A</mi><mi>C</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>B</mi><mi>C</mi></mrow><mo>^</mo></mover></math></p> <p>M&agrave;&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mrow><mi>A</mi><mi>B</mi><mi>C</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>C</mi><mi>B</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mi>n</mi><mi>&#234;</mi><mi>n</mi><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>B</mi><mi>M</mi><mo>&#160;</mo></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>C</mi><mi>N</mi></mrow><mo>^</mo></mover></math></p> <p>&nbsp;</p> <p>X&eacute;t&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8710;</mo><mi>A</mi><mi>B</mi><mi>C</mi><mo>&#160;</mo><mi>v</mi><mi>&#224;</mi><mo>&#160;</mo><mo>&#8710;</mo><mi>A</mi><mi>C</mi><mi>N</mi><mo>,</mo><mo>&#160;</mo><mi>c</mi><mi>&#243;</mi><mo>:</mo></math>&nbsp;</p> <p>AB = AC&nbsp;</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mrow><mi>A</mi><mi>B</mi><mi>M</mi><mo>&#160;</mo></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>C</mi><mi>N</mi></mrow><mo>^</mo></mover><mo>&#160;</mo></math> (cmt)</p> <p>BM = CN (gt)</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo><mo>&#8710;</mo><mi>A</mi><mi>B</mi><mi>C</mi><mo>&#160;</mo><mi>v</mi><mi>&#224;</mi><mo>&#160;</mo><mo>&#8710;</mo><mi>A</mi><mi>C</mi><mi>N</mi><mo>&#160;</mo><mo>(</mo><mi>c</mi><mo>&#160;</mo><mo>-</mo><mo>&#160;</mo><mi>g</mi><mo>&#160;</mo><mo>-</mo><mo>&#160;</mo><mi>c</mi><mo>)</mo></math></p> <p>&nbsp;</p> <p>c) Do&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8710;</mo><mi>A</mi><mi>B</mi><mi>M</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mo>&#8710;</mo><mi>A</mi><mi>C</mi><mi>N</mi><mo>&#160;</mo><mo>(</mo><mi>c</mi><mo>&#160;</mo><mo>-</mo><mi>g</mi><mo>&#160;</mo><mo>-</mo><mi>c</mi><mo>)</mo><mo>&#160;</mo><mo>&#8658;</mo><mo>&#160;</mo><mover><mrow><mi>B</mi><mi>A</mi><mi>M</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mover><mrow><mi>C</mi><mi>A</mi><mi>N</mi></mrow><mo>^</mo></mover></math> (2 g&oacute;c tương ứng)</p> <p>X&eacute;t&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8710;</mo><mi>B</mi><mi>A</mi><mi>I</mi></math> vu&ocirc;ng tại I v&agrave;&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8710;</mo><mi>C</mi><mi>A</mi><mi>K</mi></math> vu&ocirc;ng tại A:</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mrow><mi>B</mi><mi>A</mi><mi>I</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mover><mrow><mi>C</mi><mi>A</mi><mi>K</mi></mrow><mo>^</mo></mover></math> (cmt)</p> <p>AB = AC (cmt)</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo><mo>&#160;</mo><mo>&#8710;</mo><mi>B</mi><mi>A</mi><mi>I</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mo>&#8710;</mo><mi>C</mi><mi>A</mi><mi>N</mi></math> (cạnh huyền - g&oacute;c nhọn)</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo></math> AI = AK (2 cạnh tương ứng).</p> <p>&nbsp;</p> <p>Ta c&oacute;:</p> <p>∆AIK: AI = AK <math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo></math> ∆AIK c&acirc;n tại A.</p> <p>∆ABM = ∆ACN: AM = AN (2 cạnh tương ứng).</p> <p>∆ABM: AM = AN<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo></math> ∆AMN c&acirc;n tại A.</p> <p>&nbsp;</p> <p>X&eacute;t&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8710;</mo><mi>A</mi><mi>M</mi><mi>N</mi><mo>&#160;</mo><mi>c</mi><mi>&#226;</mi><mi>n</mi><mo>&#160;</mo><mi>t</mi><mi>&#7841;</mi><mi>i</mi><mo>&#160;</mo><mi>A</mi><mo>:</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>M</mi><mi>N</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>N</mi><mi>M</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mover><mrow><mi>M</mi><mi>A</mi><mi>N</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>180</mn><mo>&#176;</mo></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo><mn>2</mn><mover><mrow><mi>A</mi><mi>M</mi><mi>N</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mover><mrow><mi>M</mi><mi>A</mi><mi>N</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>180</mn><mo>&#176;</mo><mo>&#160;</mo><mo>&#8658;</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>M</mi><mi>N</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><mn>180</mn><mo>&#176;</mo><mo>&#160;</mo><mo>-</mo><mo>&#160;</mo><mover><mrow><mi>M</mi><mi>A</mi><mi>N</mi></mrow><mo>^</mo></mover></mrow><mn>2</mn></mfrac><mo>&#160;</mo><mo>(</mo><mn>1</mn><mo>)</mo></math></p> <p>X&eacute;t ∆AIK c&acirc;n tại A:&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mrow><mi>A</mi><mi>I</mi><mi>K</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>K</mi><mi>I</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mover><mrow><mi>I</mi><mi>A</mi><mi>K</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>180</mn><mo>&#176;</mo></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo><mn>2</mn><mover><mrow><mi>A</mi><mi>I</mi><mi>K</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mo>&#160;</mo><mover><mrow><mi>I</mi><mi>A</mi><mi>K</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>180</mn><mo>&#176;</mo><mo>&#160;</mo><mo>&#8658;</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>I</mi><mi>K</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><mn>180</mn><mo>&#176;</mo><mo>&#160;</mo><mo>-</mo><mo>&#160;</mo><mover><mrow><mi>I</mi><mi>A</mi><mi>K</mi></mrow><mo>^</mo></mover></mrow><mn>2</mn></mfrac><mo>&#160;</mo><mfenced><mn>2</mn></mfenced></math></p> <p>Từ (1) v&agrave; (2)&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>I</mi><mi>K</mi></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mover><mrow><mi>A</mi><mi>M</mi><mi>N</mi></mrow><mo>^</mo></mover></math></p> <p>M&agrave; hai g&oacute;c n&agrave;y ở vị tr&iacute; đồng vị n&ecirc;n IK // MN (đpcm)</p> <p>&nbsp;</p> <p>&nbsp;</p> <p>&nbsp;</p>
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