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Hướng dẫn giải Bài 41 (Trang 129 SGK Toán 9 Hình học, Tập 2)
<p>a) Ta c&oacute;: <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mrow><mi>A</mi><mi>O</mi><mi>C</mi></mrow><mo>^</mo></mover></math> =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mrow><mi>B</mi><mi>D</mi><mi>O</mi></mrow><mo>^</mo></mover></math> (c&ugrave;ng phụ&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mrow><mi>B</mi><mi>O</mi><mi>D</mi></mrow><mo>^</mo></mover></math>)</p> <p>C&aacute;c tam gi&aacute;c vu&ocirc;ng AOC v&agrave; BDO c&oacute; một g&oacute;c nhọn bằng nhau:&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mrow><mi>A</mi><mi>O</mi><mi>C</mi><mo>&#160;</mo></mrow><mo>^</mo></mover><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mover><mrow><mi>B</mi><mi>D</mi><mi>O</mi></mrow><mo>^</mo></mover></math> n&ecirc;n ch&uacute;ng đồng dạng, ta c&oacute;:&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mi>A</mi><mi>C</mi></mrow><mrow><mi>A</mi><mi>O</mi></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><mi>O</mi><mi>B</mi></mrow><mrow><mi>B</mi><mi>D</mi></mrow></mfrac></math> hay&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mi>A</mi><mi>C</mi></mrow><mi>a</mi></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mi>b</mi><mrow><mi>B</mi><mi>D</mi></mrow></mfrac></math>, suy ra:</p> <p style="text-align: left;">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; AC . BD = ab (kh&ocirc;ng đổi)</p> <p>b) Khi&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mrow><mi>A</mi><mi>O</mi><mi>C</mi></mrow><mo>^</mo></mover></math> = <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mn>60</mn><mi>o</mi></msup></math> th&igrave;&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8710;</mo></math>AOC l&agrave; nửa tam gi&aacute;c đều, cạnh OC, chiều cao AC.</p> <p>Vậy: OC = 2AO = 2a ; AC =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mi>O</mi><mi>C</mi><msqrt><mn>3</mn></msqrt></mrow><mn>2</mn></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mi>a</mi><msqrt><mn>3</mn></msqrt></math></p> <p>Thay gi&aacute; trị n&agrave;y v&agrave;o (*) ta c&oacute; BD =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mi>b</mi><msqrt><mn>3</mn></msqrt></mrow><mn>3</mn></mfrac></math></p> <p>n&ecirc;n:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>S</mi><mrow><mi>A</mi><mi>B</mi><mi>C</mi><mi>D</mi></mrow></msub></math> =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mi>A</mi><mi>C</mi><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mi>B</mi><mi>D</mi></mrow><mn>2</mn></mfrac><mo>&#160;</mo></math>. AB</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mi>a</mi><msqrt><mn>3</mn></msqrt><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mstyle displaystyle="true"><mfrac><mrow><mi>b</mi><msqrt><mn>3</mn></msqrt></mrow><mn>3</mn></mfrac></mstyle></mrow><mn>2</mn></mfrac></math> (a + b) =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mn>3</mn><mi>a</mi><msqrt><mn>3</mn></msqrt><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mi>b</mi><msqrt><mn>3</mn></msqrt></mrow><mn>6</mn></mfrac></math> (a + b)</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; = <math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><msqrt><mn>3</mn></msqrt><mn>6</mn></mfrac></math> (3a + b)(a + b) =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><msqrt><mn>3</mn></msqrt><mn>6</mn></mfrac></math> (3<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>a</mi><mn>2</mn></msup></math> +&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>b</mi><mn>2</mn></msup></math> + 4ab)</p> <p>c) Khi quay h&igrave;nh vẽ xung quanh cạnh AB: <math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8710;</mo></math>AOC tạo n&ecirc;n h&igrave;nh n&oacute;n, b&aacute;n k&iacute;nh đ&aacute;y l&agrave; AC, chiều cao AO;&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mo>&#8710;</mo></math>BOD tạo n&ecirc;n h&igrave;nh n&oacute;n, b&aacute;n k&iacute;nh đ&aacute;y BD, chiều cao OB.</p> <p>Thể t&iacute;ch h&igrave;nh n&oacute;n b&aacute;n k&iacute;nh đ&aacute;y AC l&agrave;:</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;V =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mn>1</mn><mn>3</mn></mfrac><msup><mi>&#960;AC</mi><mn>2</mn></msup></math>.AO</p> <p>Thể t&iacute;ch h&igrave;nh n&oacute;n b&aacute;n k&iacute;nh đ&aacute;y BD l&agrave;:</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>V</mi><mn>2</mn></msub></math> =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mn>1</mn><mn>3</mn></mfrac><msup><mi>&#960;BD</mi><mn>2</mn></msup></math>.OB</p> <p>Tỉ số thể t&iacute;ch l&agrave;:</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><msub><mi>V</mi><mn>1</mn></msub><msub><mi>V</mi><mn>2</mn></msub></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><mstyle displaystyle="true"><mfrac><mn>1</mn><mn>3</mn></mfrac></mstyle><msup><mi>&#960;AC</mi><mn>2</mn></msup><mo>.</mo><mi>AO</mi></mrow><mrow><mstyle displaystyle="true"><mfrac><mn>1</mn><mn>3</mn></mfrac></mstyle><msup><mi>&#960;BD</mi><mn>2</mn></msup><mo>.</mo><mi>OB</mi></mrow></mfrac></math> =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mi>A</mi><msup><mi>C</mi><mn>2</mn></msup><mo>.</mo><mi>A</mi><mi>O</mi></mrow><mrow><mi>B</mi><msup><mi>D</mi><mn>2</mn></msup><mo>.</mo><mi>O</mi><mi>B</mi></mrow></mfrac></math> =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><msup><mrow><mo>(</mo><mi>a</mi><msqrt><mn>3</mn></msqrt><mo>)</mo></mrow><mn>2</mn></msup><mo>.</mo><mi>a</mi></mrow><mrow><msup><mfenced><mstyle displaystyle="true"><mfrac><mrow><mi>b</mi><msqrt><mn>3</mn></msqrt></mrow><mn>3</mn></mfrac></mstyle></mfenced><mn>2</mn></msup><mo>.</mo><mi>b</mi></mrow></mfrac></math> =&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mn>3</mn><msup><mi>a</mi><mn>3</mn></msup></mrow><mstyle displaystyle="true"><mfrac><msup><mi>b</mi><mn>3</mn></msup><mn>3</mn></mfrac></mstyle></mfrac></math> = 9<math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><msup><mi>a</mi><mn>3</mn></msup><msup><mi>b</mi><mn>3</mn></msup></mfrac></math></p>
Hướng dẫn Giải Bài 41 (Trang 129, SGK Toán Hình học 9, Tập 2)
GV: GV colearn
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Hướng dẫn Giải Bài 41 (Trang 129, SGK Toán Hình học 9, Tập 2)
GV: GV colearn