Bài 5: Hệ số góc của đường thẳng <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>y</mi><mo>=</mo><mi>a</mi><mi>x</mi><mo>+</mo><mi>b</mi><mo>&#160;</mo><mfenced><mrow><mi>a</mi><mo>&#160;</mo><mo>&#8800;</mo><mn>0</mn></mrow></mfenced></math>
Hướng dẫn giải Bài 31 (Trang 59 SGK Toán 9, Tập 1)
<p><strong>B&agrave;i 31 (Trang 59 SGK To&aacute;n 9, Tập 1):</strong></p> <p>a) Vẽ đồ thị của c&aacute;c h&agrave;m số <math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal">y</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mi mathvariant="normal">x</mi><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><mn>1</mn><mo>;</mo><mo>&#160;</mo><mi mathvariant="normal">y</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mn>1</mn><msqrt><mn>3</mn></msqrt></mfrac><mi mathvariant="normal">x</mi><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><msqrt><mn>3</mn></msqrt><mo>;</mo><mo>&#160;</mo><mi mathvariant="normal">y</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><mn>3</mn></msqrt><mi mathvariant="normal">x</mi><mo>&#160;</mo><mo>-</mo><mo>&#160;</mo><msqrt><mn>3</mn></msqrt><mo>.</mo></math></p> <p>b) Gọi&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal">&#945;</mi><mo>,</mo><mo>&#160;</mo><mi mathvariant="normal">&#946;</mi><mo>,</mo><mo>&#160;</mo><mi mathvariant="normal">&#947;</mi></math> lần lượt l&agrave; c&aacute;c g&oacute;c tạo bởi c&aacute;c đường thẳng tr&ecirc;n v&agrave; trục Ox.</p> <p>Chứng minh rằng:&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>t</mi><mi>g</mi><mi>&#945;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>1</mn><mo>,</mo><mo>&#160;</mo><mi>t</mi><mi>g</mi><mi>&#946;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mn>1</mn><msqrt><mn>3</mn></msqrt></mfrac><mo>,</mo><mo>&#160;</mo><mi>t</mi><mi>g</mi><mi>&#947;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><mn>3</mn></msqrt><mo>.</mo></math></p> <p>T&iacute;nh số đo c&aacute;c g&oacute;c&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>&#945;</mi><mo>,</mo><mo>&#160;</mo><mi>&#946;</mi><mo>,</mo><mo>&#160;</mo><mi>&#947;</mi></math>.</p> <p>&nbsp;</p> <p><strong><span style="text-decoration: underline;"><em>Hướng dẫn giải:</em></span></strong></p> <p>a)</p> <p>- Vẽ đồ thị h&agrave;m số y = x + 1</p> <p>Cho x = 0 =&gt; y = 1 =&gt; Ta c&oacute; điểm A(0;1)</p> <p>Cho y = 0 =&gt; x = -1 =&gt; Ta c&oacute; điểm B(-1;0)</p> <p>Đồ thị h&agrave;m số y = x + 1 l&agrave; đường thẳng đi qua điểm A v&agrave; B.</p> <p>- Vẽ đồ thị h&agrave;m số <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>y</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mn>1</mn><msqrt><mn>3</mn></msqrt></mfrac><mi>x</mi><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><msqrt><mn>3</mn></msqrt></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>Cho</mi><mo>&#160;</mo><mi mathvariant="normal">x</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>0</mn><mo>&#160;</mo><mo>=</mo><mo>&#62;</mo><mo>&#160;</mo><mi mathvariant="normal">y</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><mn>3</mn></msqrt><mo>&#160;</mo><mo>=</mo><mo>&#62;</mo><mo>&#160;</mo><mi>Ta</mi><mo>&#160;</mo><mi>c&#243;</mi><mo>&#160;</mo><mi>&#273;i&#7875;m</mi><mo>&#160;</mo><mi mathvariant="normal">C</mi><mo>(</mo><mn>0</mn><mo>;</mo><msqrt><mn>3</mn></msqrt><mo>)</mo></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>Cho</mi><mo>&#160;</mo><mi mathvariant="normal">y</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>0</mn><mo>&#160;</mo><mo>=</mo><mo>&#62;</mo><mo>&#160;</mo><mi mathvariant="normal">x</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mo>-</mo><mn>3</mn><mo>&#160;</mo><mo>=</mo><mo>&#62;</mo><mo>&#8201;</mo><mi>Ta</mi><mo>&#160;</mo><mi>c&#243;</mi><mo>&#160;</mo><mi>&#273;i&#7875;m</mi><mo>&#160;</mo><mi mathvariant="normal">D</mi><mo>(</mo><mo>-</mo><mn>3</mn><mo>;</mo><mn>0</mn><mo>)</mo></math></p> <p>Đồ thị h&agrave;m số&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>y</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mn>1</mn><msqrt><mn>3</mn></msqrt></mfrac><mi>x</mi><mo>&#160;</mo><mo>+</mo><mo>&#160;</mo><msqrt><mn>3</mn></msqrt></math> l&agrave; đường thẳng đi qua điểm C v&agrave; D.</p> <p>- Vẽ đồ thị h&agrave;m số <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>y</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><mn>3</mn></msqrt><mi>x</mi><mo>&#160;</mo><mo>-</mo><mo>&#160;</mo><msqrt><mn>3</mn></msqrt></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>Cho</mi><mo>&#160;</mo><mi mathvariant="normal">x</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>0</mn><mo>&#160;</mo><mo>=</mo><mo>&#62;</mo><mo>&#160;</mo><mi mathvariant="normal">y</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mo>-</mo><msqrt><mn>3</mn></msqrt><mo>&#160;</mo><mo>=</mo><mo>&#62;</mo><mo>&#160;</mo><mi>Ta</mi><mo>&#160;</mo><mi>c&#243;</mi><mo>&#160;</mo><mi>&#273;i&#7875;m</mi><mo>&#160;</mo><mi mathvariant="normal">E</mi><mo>(</mo><mn>0</mn><mo>;</mo><mo>-</mo><msqrt><mn>3</mn></msqrt><mo>)</mo></math></p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>Cho</mi><mo>&#160;</mo><mi mathvariant="normal">y</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>0</mn><mo>&#160;</mo><mo>=</mo><mo>&#62;</mo><mo>&#160;</mo><mi mathvariant="normal">x</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>1</mn><mo>&#160;</mo><mo>=</mo><mo>&#62;</mo><mo>&#8201;</mo><mi>Ta</mi><mo>&#160;</mo><mi>c&#243;</mi><mo>&#160;</mo><mi>&#273;i&#7875;m</mi><mo>&#160;</mo><mi mathvariant="normal">F</mi><mo>(</mo><mn>1</mn><mo>;</mo><mn>0</mn><mo>)</mo></math></p> <p>Đồ thị h&agrave;m số&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>y</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><mn>3</mn></msqrt><mi>x</mi><mo>&#160;</mo><mo>-</mo><mo>&#160;</mo><msqrt><mn>3</mn></msqrt></math> l&agrave; đường thẳng đi qua hai điểm E v&agrave; F.</p> <p>Vẽ đồ thị c&aacute;c h&agrave;m số tr&ecirc;n như sau:</p> <p><img class="wscnph" style="max-width: 100%;" src="https://static.colearn.vn:8413/v1.0/upload/library/26102022/bai-31-trand-59-sdk-toan-9-tap-1-3-WjRa6q.jpg" /></p> <p>b) Gọi O l&agrave; gốc tọa độ</p> <p>Ta c&oacute;:</p> <p><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>tan</mi><mo>&#160;</mo><mi>&#945;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><mi>O</mi><mi>A</mi></mrow><mrow><mi>O</mi><mi>B</mi></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mfenced open="|" close="|"><mn>1</mn></mfenced><mfenced open="|" close="|"><mrow><mo>-</mo><mn>1</mn></mrow></mfenced></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>1</mn><mo>&#160;</mo><mo>&#8658;</mo><mo>&#160;</mo><mi>&#945;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>45</mn><mo>&#176;</mo><mspace linebreak="newline"/><mi>tan</mi><mo>&#160;</mo><mi>&#946;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><mi>O</mi><mi>C</mi></mrow><mrow><mi>O</mi><mi>D</mi></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mfenced open="|" close="|"><msqrt><mn>3</mn></msqrt></mfenced><mfenced open="|" close="|"><mrow><mo>-</mo><msqrt><mn>3</mn></msqrt></mrow></mfenced></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mn>1</mn><msqrt><mn>3</mn></msqrt></mfrac><mo>&#160;</mo><mo>&#8658;</mo><mo>&#160;</mo><mi>&#946;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>30</mn><mo>&#176;</mo><mspace linebreak="newline"/><mi>tan</mi><mo>&#160;</mo><mi>&#947;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mrow><mi>O</mi><mi>E</mi></mrow><mrow><mi>O</mi><mi>F</mi></mrow></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mfrac><mfenced open="|" close="|"><mrow><mo>-</mo><msqrt><mn>3</mn></msqrt></mrow></mfenced><mn>1</mn></mfrac><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><msqrt><mn>3</mn></msqrt><mo>&#160;</mo><mo>&#8658;</mo><mo>&#160;</mo><mi>&#947;</mi><mo>&#160;</mo><mo>=</mo><mo>&#160;</mo><mn>60</mn><mo>&#176;</mo></math></p>
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