Bài 5: Dấu hiệu nhận biết tiếp tuyến của đường tròn
Hướng dẫn giải Bài 24 (Trang 111 SGK Toán Hình học 9, Tập 1)
<p>Cho đường tr&ograve;n (O), d&acirc;y AB kh&aacute;c đường k&iacute;nh. Qua O kẻ đường vu&ocirc;ng g&oacute;c với AB, cắt tiếp tuyến tại A của đường tr&ograve;n ở điểm C</p> <p>a) Chứng minh rằng CB l&agrave; tiếp tuyến của đường tr&ograve;n</p> <p>b) Cho b&aacute;n k&iacute;nh của đường tr&ograve;n bằng 15cm, AB = 24 cm. T&iacute;nh độ d&agrave;i OC</p> <p>Giải</p> <p><img class="wscnph" src="https://static.colearn.vn:8413/v1.0/upload/library/16022022/anh-chup-man-hinh-2022-02-11-luc-152059-unpZfX.png" /></p> <p>a) Gọi H l&agrave; giao điểm của OC v&agrave; AB</p> <p><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span>OH&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8869;</mo></math>AB<span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo></math><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span>AH = HB</p> <p>Vậy OC l&agrave; đường trung trực của đoạn AB</p> <p><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span>&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo></math><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span>AC = BC</p> <p>X&eacute;t &nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#9651;</mo></math>OAC v&agrave;&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#9651;</mo></math>OBC c&oacute;:</p> <p>OA = OB (= R), <span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span>AC = BC;<span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span>OC (cạnh chung)</p> <p>Do đ&oacute;&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#9651;</mo></math>OAC =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#9651;</mo></math>OBC (c.c.c)</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo></math><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mtext>OAC</mtext><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mover><mtext>OBC</mtext><mo>^</mo></mover></math></p> <p>N&ecirc;n<span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mtext>OBC</mtext><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></math><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span>CB l&agrave; tiếp tuyến của đường tr&ograve;n.</p> <p>b)<span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span>OH&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8869;</mo></math>AB (gt)&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8658;</mo></math>&nbsp;AH = HB =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mtext>AB</mtext><mn>2</mn></mfrac></math>&nbsp;= 12 (cm)</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#9651;</mo></math>OAH vu&ocirc;ng tại H, theo định l&iacute; Py-ta-go c&oacute;:</p> <p><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mtext>OH</mtext><mn>2</mn></msup><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><msup><mtext>AH</mtext><mn>2</mn></msup><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msup><mtext>OA</mtext><mn>2</mn></msup></math></p> <p><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><span class="mce-nbsp-wrap" contenteditable="false">&nbsp;&nbsp;&nbsp;</span><math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mtext>OH</mtext><mn>2</mn></msup></math>&nbsp;=&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mn>15</mn><mn>2</mn></msup><mo>&#160;</mo><mo>-</mo><mo>&#160;</mo><msup><mn>12</mn><mn>2</mn></msup><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>81</mn><mo>&#160;</mo><mo>&#8658;</mo><mo>&#160;</mo><mtext>OH</mtext><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><mn>81</mn></msqrt><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>9</mn><mo>&#160;</mo><mo>(</mo><mtext>cm</mtext><mo>)</mo></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#9651;</mo></math>OAC vu&ocirc;ng tại A, AH l&agrave; đường cao n&ecirc;n OH . OC =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mtext>OA</mtext><mn>2</mn></msup></math></p> <p>Do đ&oacute; 9 . OC =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mn>15</mn><mrow><mn>2</mn><mo>&#160;</mo></mrow></msup><mo>&#160;</mo><mo>&#8658;</mo><mo>&#160;</mo><mtext>OC</mtext><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><msup><mn>15</mn><mn>2</mn></msup><mrow><mn>9</mn><mo>&#160;</mo></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>25</mn><mo>&#160;</mo><mo>(</mo><mtext>cm</mtext><mo>)</mo></math></p>
Hướng dẫn Giải Bài 24 (Trang 111, SGK Toán Hình học 9, Tập 1)
GV: GV colearn
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Hướng dẫn Giải Bài 24 (Trang 111, SGK Toán Hình học 9, Tập 1)
GV: GV colearn