Bài 3: Liên hệ giữa phép nhân và phép khai phương
Hướng dẫn giải Bài 20 (Trang 15 SGK Toán 9, Tập 1)
<div data-v-4ef816dc=""><strong>B&agrave;i 20 (Trang 15 SGK To&aacute;n 9, Tập 1):</strong></div> <p>R&uacute;t gọn c&aacute;c biểu thức sau:</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>a</mi><mo>)</mo><mo>&#160;</mo><msqrt><mfrac><mrow><mn>2</mn><mi>a</mi></mrow><mn>3</mn></mfrac></msqrt><mo>&#160;</mo><mo>.</mo><msqrt><mfrac><mrow><mn>3</mn><mi>a</mi></mrow><mn>8</mn></mfrac></msqrt><mi>v</mi><mi>&#7899;</mi><mi>i</mi><mo>&#160;</mo><mi>a</mi><mo>&#8805;</mo><mn>0</mn><mo>;</mo><mspace linebreak="newline"/><mspace linebreak="newline"/><mi>b</mi><mo>)</mo><mo>&#160;</mo><msqrt><mn>13</mn><mi>a</mi></msqrt><mo>&#160;</mo><mo>.</mo><msqrt><mfrac><mn>52</mn><mi>a</mi></mfrac></msqrt><mi>v</mi><mi>&#7899;</mi><mi>i</mi><mo>&#160;</mo><mi>a</mi><mo>&#62;</mo><mn>0</mn><mo>;</mo><mspace linebreak="newline"/><mspace linebreak="newline"/><mi>c</mi><mo>)</mo><mo>&#160;</mo><msqrt><mn>5</mn><mi>a</mi></msqrt><mo>.</mo><msqrt><mn>45</mn><mi>a</mi></msqrt><mo>-</mo><mn>3</mn><mi>a</mi><mo>&#160;</mo><mi>v</mi><mi>&#7899;</mi><mi>i</mi><mo>&#160;</mo><mi>a</mi><mo>&#8805;</mo><mn>0</mn><mo>;</mo><mspace linebreak="newline"/><mspace linebreak="newline"/><mi>d</mi><mo>)</mo><mo>&#160;</mo><msup><mfenced><mrow><mn>3</mn><mo>-</mo><mi>a</mi></mrow></mfenced><mn>2</mn></msup><mo>-</mo><msqrt><mn>0</mn><mo>,</mo><mn>2</mn></msqrt><mo>.</mo><msqrt><mn>180</mn><msup><mi>a</mi><mn>2</mn></msup></msqrt></math></p> <p>&nbsp;</p> <p><strong><span style="text-decoration: underline;"><em>Hướng dẫn giải:</em></span></strong></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal">a</mi><mo>)</mo><mo>&#160;</mo><msqrt><mfrac><mrow><mn>2</mn><mi mathvariant="normal">a</mi></mrow><mn>3</mn></mfrac></msqrt><mo>&#160;</mo><mo>.</mo><msqrt><mfrac><mrow><mn>3</mn><mi mathvariant="normal">a</mi></mrow><mn>8</mn></mfrac></msqrt><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><mfrac><mrow><mn>2</mn><mi mathvariant="normal">a</mi></mrow><mn>3</mn></mfrac><mo>.</mo><mfrac><mrow><mn>3</mn><mi mathvariant="normal">a</mi></mrow><mn>8</mn></mfrac></msqrt><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><mfrac><msup><mi mathvariant="normal">a</mi><mn>2</mn></msup><mn>4</mn></mfrac></msqrt><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><msup><mfenced><mfrac><mi mathvariant="normal">a</mi><mn>2</mn></mfrac></mfenced><mn>2</mn></msup></msqrt><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfenced open="|" close="|"><mfrac><mi mathvariant="normal">a</mi><mn>2</mn></mfrac></mfenced><mo>&#160;</mo><mo>&#160;</mo><mo>(</mo><mi>v&#236;</mi><mo>&#160;</mo><mi mathvariant="normal">a</mi><mo>&#8805;</mo><mn>0</mn><mo>)</mo><mo>.</mo></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal">b</mi><mo>)</mo><mo>&#160;</mo><msqrt><mn>13</mn><mi mathvariant="normal">a</mi></msqrt><mo>&#160;</mo><mo>.</mo><msqrt><mfrac><mn>52</mn><mi mathvariant="normal">a</mi></mfrac></msqrt><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><mn>13</mn><mi mathvariant="normal">a</mi><mo>.</mo><mfrac><mn>52</mn><mi mathvariant="normal">a</mi></mfrac></msqrt><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><mn>13</mn><mo>.</mo><mn>13</mn><mo>.</mo><mn>4</mn></msqrt><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><msup><mn>26</mn><mn>2</mn></msup></msqrt><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>26</mn><mo>&#160;</mo><mo>(</mo><mi>v&#236;</mi><mo>&#160;</mo><mo>&#160;</mo><mi mathvariant="normal">a</mi><mo>&#62;</mo><mn>0</mn><mo>)</mo><mo>.</mo></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal">c</mi><mo>)</mo><mo>&#160;</mo><msqrt><mn>5</mn><mi mathvariant="normal">a</mi></msqrt><mo>.</mo><msqrt><mn>45</mn><mi mathvariant="normal">a</mi></msqrt><mo>-</mo><mn>3</mn><mi mathvariant="normal">a</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><mn>15</mn><mi mathvariant="normal">a</mi><mo>.</mo><mn>45</mn><mi mathvariant="normal">a</mi></msqrt><mo>-</mo><mn>3</mn><mi mathvariant="normal">a</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><msup><mfenced><mrow><mn>15</mn><mi mathvariant="normal">a</mi></mrow></mfenced><mn>2</mn></msup></msqrt><mo>-</mo><mn>3</mn><mi mathvariant="normal">a</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfenced open="|" close="|"><mrow><mn>15</mn><mi mathvariant="normal">a</mi></mrow></mfenced><mo>-</mo><mn>3</mn><mi mathvariant="normal">a</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>15</mn><mi mathvariant="normal">a</mi><mo>-</mo><mn>3</mn><mi mathvariant="normal">a</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>12</mn><mi mathvariant="normal">a</mi><mo>&#160;</mo><mo>(</mo><mi>v&#236;</mi><mo>&#160;</mo><mi mathvariant="normal">a</mi><mo>&#8805;</mo><mn>0</mn><mo>)</mo><mo>.</mo></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal">d</mi><mo>)</mo><mo>&#160;</mo><msup><mfenced><mrow><mn>3</mn><mo>-</mo><mi mathvariant="normal">a</mi></mrow></mfenced><mn>2</mn></msup><mo>-</mo><msqrt><mn>0</mn><mo>,</mo><mn>2</mn></msqrt><mo>.</mo><msqrt><mn>180</mn><msup><mi mathvariant="normal">a</mi><mn>2</mn></msup></msqrt><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msup><mfenced><mrow><mn>3</mn><mo>-</mo><mi mathvariant="normal">a</mi></mrow></mfenced><mn>2</mn></msup><mo>-</mo><msqrt><mn>0</mn><mo>,</mo><mn>2</mn><mo>.</mo><mn>180</mn><msup><mi mathvariant="normal">a</mi><mn>2</mn></msup></msqrt><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msup><mfenced><mrow><mn>3</mn><mo>-</mo><mi mathvariant="normal">a</mi></mrow></mfenced><mn>2</mn></msup><mo>-</mo><msqrt><msup><mfenced><mrow><mn>6</mn><mi mathvariant="normal">a</mi></mrow></mfenced><mn>2</mn></msup></msqrt><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>9</mn><mo>-</mo><mn>6</mn><mi mathvariant="normal">a</mi><mo>+</mo><msup><mi mathvariant="normal">a</mi><mn>2</mn></msup><mo>-</mo><mfenced open="|" close="|"><mrow><mn>6</mn><mi mathvariant="normal">a</mi></mrow></mfenced></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>+</mo><mo>)</mo><mo>&#160;</mo><mi>N</mi><mi>&#7871;</mi><mi>u</mi><mo>&#160;</mo><mi>a</mi><mo>&#160;</mo><mo>&#8805;</mo><mo>&#160;</mo><mn>0</mn><mo>&#160;</mo><mo>&#8658;</mo><mo>&#160;</mo><mi>b</mi><mi>i</mi><mi>&#7875;</mi><mi>u</mi><mo>&#160;</mo><mi>t</mi><mi>h</mi><mi>&#7913;</mi><mi>c</mi><mo>&#160;</mo><mi>t</mi><mi>r</mi><mi>&#234;</mi><mi>n</mi><mo>&#160;</mo><mo>=</mo><mn>9</mn><mo>-</mo><mn>6</mn><mi mathvariant="normal">a</mi><mo>+</mo><msup><mi mathvariant="normal">a</mi><mn>2</mn></msup><mo>-</mo><mn>6</mn><mi mathvariant="normal">a</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>9</mn><mo>-</mo><mn>12</mn><mi mathvariant="normal">a</mi><mo>+</mo><msup><mi mathvariant="normal">a</mi><mn>2</mn></msup><mspace linebreak="newline"/><mspace linebreak="newline"/><mo>+</mo><mo>)</mo><mo>&#160;</mo><mi>N&#7871;u</mi><mo>&#160;</mo><mi mathvariant="normal">a</mi><mo>&#160;</mo><mo>&#60;</mo><mo>&#160;</mo><mn>0</mn><mo>&#160;</mo><mo>&#8658;</mo><mo>&#160;</mo><mi>bi&#7875;u</mi><mo>&#160;</mo><mi>th&#7913;c</mi><mo>&#160;</mo><mi>tr&#234;n</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>9</mn><mo>-</mo><mn>6</mn><mi mathvariant="normal">a</mi><mo>+</mo><msup><mi mathvariant="normal">a</mi><mn>2</mn></msup><mo>+</mo><mn>6</mn><mi mathvariant="normal">a</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>9</mn><mo>&#160;</mo><mo>+</mo><mo>&#8201;</mo><msup><mi mathvariant="normal">a</mi><mn>2</mn></msup></math></p>
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