Bài 3: Tính Chất Đường Phân Giác Của Tam Giác
Hướng dẫn giải Bài 21 (Trang 68 SGK Toán Hình học 8, Tập 2)
<p><strong class="content_question">Đề bài</strong></p>
<p>a) Cho tam giác <span id="MathJax-Element-1-Frame" class="mjx-chtml MathJax_CHTML" style="margin: 0px; padding: 1px 0px; display: inline-block; line-height: 0; text-indent: 0px; text-align: left; text-transform: none; font-style: normal; font-weight: normal; font-size: 16.94px; letter-spacing: normal; overflow-wrap: normal; word-spacing: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>A</mi><mi>B</mi><mi>C</mi></math>"><span id="MJXc-Node-1" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-2" class="mjx-mrow"><span id="MJXc-Node-3" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">A</span></span><span id="MJXc-Node-4" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">B</span></span><span id="MJXc-Node-5" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">C</span></span></span></span></span> với đường trung tuyến <span id="MathJax-Element-2-Frame" class="mjx-chtml MathJax_CHTML" style="margin: 0px; padding: 1px 0px; display: inline-block; line-height: 0; text-indent: 0px; text-align: left; text-transform: none; font-style: normal; font-weight: normal; font-size: 16.94px; letter-spacing: normal; overflow-wrap: normal; word-spacing: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>A</mi><mi>M</mi></math>"><span id="MJXc-Node-6" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-7" class="mjx-mrow"><span id="MJXc-Node-8" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">A</span></span></span></span><span class="MJX_Assistive_MathML" role="presentation"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>M</mi></math></span></span> và đường phân giác <span id="MathJax-Element-3-Frame" class="mjx-chtml MathJax_CHTML" style="margin: 0px; padding: 1px 0px; display: inline-block; line-height: 0; text-indent: 0px; text-align: left; text-transform: none; font-style: normal; font-weight: normal; font-size: 16.94px; letter-spacing: normal; overflow-wrap: normal; word-spacing: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>A</mi><mi>D</mi></math>"><span id="MJXc-Node-10" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-11" class="mjx-mrow"><span id="MJXc-Node-12" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">A</span></span></span></span><span class="MJX_Assistive_MathML" role="presentation"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>D</mi></math></span></span>. Tính diện tích tam giác <span id="MathJax-Element-4-Frame" class="mjx-chtml MathJax_CHTML" style="margin: 0px; padding: 1px 0px; display: inline-block; line-height: 0; text-indent: 0px; text-align: left; text-transform: none; font-style: normal; font-weight: normal; font-size: 16.94px; letter-spacing: normal; overflow-wrap: normal; word-spacing: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>A</mi><mi>D</mi><mi>M</mi></math>"><span id="MJXc-Node-14" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-15" class="mjx-mrow"><span id="MJXc-Node-16" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">A</span></span><span id="MJXc-Node-17" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">D</span></span><span id="MJXc-Node-18" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">M</span></span></span></span></span>, biết <span id="MathJax-Element-5-Frame" class="mjx-chtml MathJax_CHTML" style="margin: 0px; padding: 1px 0px; display: inline-block; line-height: 0; text-indent: 0px; text-align: left; text-transform: none; font-style: normal; font-weight: normal; font-size: 16.94px; letter-spacing: normal; overflow-wrap: normal; word-spacing: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>A</mi><mi>B</mi><mo>=</mo><mi>m</mi><mo>,</mo><mi>A</mi><mi>C</mi><mo>=</mo><mi>n</mi><mspace width="thickmathspace" /><mo stretchy="false">(</mo><mi>n</mi><mo>&gt;</mo><mi>m</mi><mo stretchy="false">)</mo></math>"><span id="MJXc-Node-19" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-20" class="mjx-mrow"><span id="MJXc-Node-21" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">A</span></span><span id="MJXc-Node-22" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">B</span></span><span id="MJXc-Node-23" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-24" class="mjx-mi MJXc-space3"><span class="mjx-char MJXc-TeX-math-I">m</span></span><span id="MJXc-Node-25" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">,</span></span><span id="MJXc-Node-26" class="mjx-mi MJXc-space1"><span class="mjx-char MJXc-TeX-math-I">A</span></span><span id="MJXc-Node-27" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">C</span></span><span id="MJXc-Node-28" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-29" class="mjx-mi MJXc-space3"><span class="mjx-char MJXc-TeX-math-I">n</span></span><span id="MJXc-Node-30" class="mjx-mspace"></span><span id="MJXc-Node-31" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">(</span></span><span id="MJXc-Node-32" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">n</span></span><span id="MJXc-Node-33" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">></span></span><span id="MJXc-Node-34" class="mjx-mi MJXc-space3"><span class="mjx-char MJXc-TeX-math-I">m</span></span><span id="MJXc-Node-35" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">)</span></span></span></span></span> và diện tích của tam giác <span id="MathJax-Element-6-Frame" class="mjx-chtml MathJax_CHTML" style="margin: 0px; padding: 1px 0px; display: inline-block; line-height: 0; text-indent: 0px; text-align: left; text-transform: none; font-style: normal; font-weight: normal; font-size: 16.94px; letter-spacing: normal; overflow-wrap: normal; word-spacing: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>A</mi><mi>B</mi><mi>C</mi></math>"><span id="MJXc-Node-36" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-37" class="mjx-mrow"><span id="MJXc-Node-38" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">A</span></span><span id="MJXc-Node-39" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">B</span></span><span id="MJXc-Node-40" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">C</span></span></span></span></span> là <span id="MathJax-Element-7-Frame" class="mjx-chtml MathJax_CHTML" style="margin: 0px; padding: 1px 0px; display: inline-block; line-height: 0; text-indent: 0px; text-align: left; text-transform: none; font-style: normal; font-weight: normal; font-size: 16.94px; letter-spacing: normal; overflow-wrap: normal; word-spacing: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>S</mi></math>"><span class="MJX_Assistive_MathML" role="presentation"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>S</mi></math></span></span>.</p>
<p>b) Cho <span id="MathJax-Element-8-Frame" class="mjx-chtml MathJax_CHTML" style="margin: 0px; padding: 1px 0px; display: inline-block; line-height: 0; text-indent: 0px; text-align: left; text-transform: none; font-style: normal; font-weight: normal; font-size: 16.94px; letter-spacing: normal; overflow-wrap: normal; word-spacing: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>n</mi><mo>=</mo><mn>7</mn><mi>c</mi><mi>m</mi><mo>,</mo><mi>m</mi><mo>=</mo><mn>3</mn><mi>c</mi><mi>m</mi></math>"><span id="MJXc-Node-44" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-45" class="mjx-mrow"><span id="MJXc-Node-46" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">n</span></span><span id="MJXc-Node-47" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-48" class="mjx-mn MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">7</span></span><span id="MJXc-Node-49" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">c</span></span><span id="MJXc-Node-50" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">m</span></span><span id="MJXc-Node-51" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">,</span></span><span id="MJXc-Node-52" class="mjx-mi MJXc-space1"><span class="mjx-char MJXc-TeX-math-I">m</span></span><span id="MJXc-Node-53" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-54" class="mjx-mn MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">3</span></span><span id="MJXc-Node-55" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">c</span></span><span id="MJXc-Node-56" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">m</span></span></span></span></span>. Hỏi diện tích tam giác <span id="MathJax-Element-9-Frame" class="mjx-chtml MathJax_CHTML" style="margin: 0px; padding: 1px 0px; display: inline-block; line-height: 0; text-indent: 0px; text-align: left; text-transform: none; font-style: normal; font-weight: normal; font-size: 16.94px; letter-spacing: normal; overflow-wrap: normal; word-spacing: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>A</mi><mi>D</mi><mi>M</mi></math>"><span class="MJX_Assistive_MathML" role="presentation"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>A</mi><mi>D</mi><mi>M</mi></math></span></span> chiếm bao nhiêu phần trăm diện tích tam giác <span id="MathJax-Element-10-Frame" class="mjx-chtml MathJax_CHTML" style="margin: 0px; padding: 1px 0px; display: inline-block; line-height: 0; text-indent: 0px; text-align: left; text-transform: none; font-style: normal; font-weight: normal; font-size: 16.94px; letter-spacing: normal; overflow-wrap: normal; word-spacing: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>A</mi><mi>B</mi><mi>C</mi></math>"><span id="MJXc-Node-62" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-63" class="mjx-mrow"><span id="MJXc-Node-64" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">A</span></span><span id="MJXc-Node-65" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">B</span></span><span id="MJXc-Node-66" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">C</span></span></span></span></span>.</p>
<p><strong class="content_detail">Lời giải chi tiết</strong></p>
<p><img src="https://img.loigiaihay.com/picture/2019/0314/bai-21-trang-68-sgk-toan-8-t2.jpg" /></p>
<p>a) Ta có AD là đường phân giác của <math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="false"><mi mathvariant="normal">Δ</mi><mo>⁢</mo><mi>A</mi><mo>⁢</mo><mi>B</mi><mo>⁢</mo><mi>C</mi></mstyle></math> (gt) nên<br /><math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="false"><mfrac><mrow><mi>B</mi><mo>⁢</mo><mi mathvariant="normal">D</mi></mrow><mrow><mi>D</mi><mo>⁢</mo><mi>C</mi></mrow></mfrac><mo>=</mo><mfrac><mrow><mi>A</mi><mo>⁢</mo><mi>B</mi></mrow><mrow><mi>A</mi><mo>⁢</mo><mi>C</mi></mrow></mfrac></mstyle></math> (Tính chất đường phân giác của tam giác) <math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="false"><mfrac><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>B</mi><mo>⁢</mo><mi>D</mi></mrow></msub><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>D</mi><mo>⁢</mo><mi>C</mi></mrow></msub></mfrac><mo>=</mo><mfrac><mrow><mi>D</mi><mo>⁢</mo><mi>B</mi></mrow><mrow><mi>D</mi><mo>⁢</mo><mi>C</mi></mrow></mfrac></mstyle></math> (do hai tam giác có chung chiều cao từ' đỉnh A)</p>
<p>Nên <math xmlns="http://www.w3.org/1998/Math/MathML"><mo>⁢</mo><mfrac><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>B</mi><mo>⁢</mo><mi>D</mi></mrow></msub><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>D</mi><mo>⁢</mo><mi>C</mi></mrow></msub></mfrac><mo>=</mo><mfrac><mrow><mi>D</mi><mo>⁢</mo><mi>B</mi></mrow><mrow><mi>D</mi><mo>⁢</mo><mi>C</mi></mrow></mfrac><mo>=</mo><mfrac><mrow><mi>A</mi><mo>⁢</mo><mi>B</mi></mrow><mrow><mi>A</mi><mo>⁢</mo><mi>C</mi></mrow></mfrac><mo>=</mo><mfrac><mi>m</mi><mi>n</mi></mfrac><mo>⁢</mo></math></p>
<p><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>⇒</mo><mfrac><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>D</mi><mo>⁢</mo><mi>C</mi></mrow></msub><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>B</mi><mo>⁢</mo><mi>D</mi></mrow></msub></mfrac><mo>=</mo><mpadded><mfrac><mi>n</mi><mi>m</mi></mfrac></mpadded><mo>⁢</mo><mspace linebreak="newline"/><mo>⇒</mo><mfrac><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>D</mi><mo>⁢</mo><mi>C</mi></mrow></msub><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>B</mi><mo>⁢</mo><mi>D</mi></mrow></msub></mfrac><mo>+</mo><mn>1</mn><mo>=</mo><mfrac><mi>n</mi><mi>m</mi></mfrac><mo>+</mo><mpadded><mn>1</mn></mpadded><mo>⁢</mo><mspace linebreak="newline"/><mo>⇒</mo><mfrac><mrow><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>D</mi><mo>⁢</mo><mi>C</mi></mrow></msub><mo>+</mo><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>B</mi><mo>⁢</mo><mi>D</mi></mrow></msub></mrow><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>B</mi><mo>⁢</mo><mi>D</mi></mrow></msub></mfrac><mo>=</mo><mpadded><mfrac><mrow><mi>n</mi><mo>+</mo><mi>m</mi></mrow><mi>m</mi></mfrac></mpadded><mo>⁢</mo><mspace linebreak="newline"/><mo>⇒</mo><mfrac><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>B</mi><mo>⁢</mo><mi>D</mi></mrow></msub><mrow><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>D</mi><mo>⁢</mo><mi>C</mi></mrow></msub><mo>+</mo><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>B</mi><mo>⁢</mo><mi>D</mi></mrow></msub></mrow></mfrac><mo>=</mo><mfrac><mi>m</mi><mrow><mi>n</mi><mo>+</mo><mi>m</mi></mrow></mfrac><mo>⁢</mo></math></p>
<p>hay <math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>B</mi><mo>⁢</mo><mi>D</mi></mrow></msub><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>B</mi><mo>⁢</mo><mi>C</mi></mrow></msub></mfrac><mo>=</mo><mpadded><mfrac><mi>m</mi><mrow><mi>n</mi><mo>+</mo><mi>m</mi></mrow></mfrac></mpadded><mo>⁢</mo><mspace linebreak="newline"/><mo>⇒</mo><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>B</mi><mo>⁢</mo><mi>D</mi></mrow></msub><mo>=</mo><mfrac><mrow><mi>m</mi><mo>⁢</mo><mi>S</mi></mrow><mrow><mi>n</mi><mo>+</mo><mi>m</mi></mrow></mfrac><mo>⁢</mo></math><br />Vì AM là trung tuyến của <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><mi>A</mi><mo>⁢</mo><mi>B</mi><mo>⁢</mo><mi>C</mi><mo>⁢</mo><mrow><mo>(</mo><mi>gt</mi><mo>)</mo></mrow><mo>⇒</mo><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>B</mi><mo>⁢</mo><mi>M</mi></mrow></msub><mo>=</mo><mfrac><mn>1</mn><mn>2</mn></mfrac><mo>⁢</mo><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>B</mi><mo>⁢</mo><mi>C</mi></mrow></msub></mrow><mo>.</mo></math><br />Có AB<AC(m<n) và AD là đường phân giác, AM là đường trung tuyến kẻ từ' A nên AD nằm giữa AB và AM.<br /><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>⇒</mo><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>D</mi><mo>⁢</mo><mi>M</mi></mrow></msub><mo>=</mo><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>B</mi><mo>⁢</mo><mi>M</mi></mrow></msub><mo>-</mo><mpadded><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>B</mi><mo>⁢</mo><mi>D</mi></mrow></msub></mpadded><mo>⁢</mo><mspace linebreak="newline"/><mo>⇒</mo><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>D</mi><mo>⁢</mo><mi>M</mi></mrow></msub><mo>=</mo><mfrac><mn>1</mn><mn>2</mn></mfrac><mo>⁢</mo><mi>S</mi><mo>-</mo><mfrac><mi>m</mi><mrow><mi>n</mi><mo>+</mo><mi>m</mi></mrow></mfrac><mo>⁢</mo><mi>S</mi><mo>=</mo><mfrac><mrow><mi>S</mi><mo>⁢</mo><mrow><mo>(</mo><mi>m</mi><mo>+</mo><mi>n</mi><mo>-</mo><mn>2</mn><mo>⁢</mo><mi>m</mi><mo>)</mo></mrow></mrow><mrow><mn>2</mn><mo>⁢</mo><mrow><mo>(</mo><mi>m</mi><mo>+</mo><mi>n</mi><mo>)</mo></mrow></mrow></mfrac><mo>⁢</mo></math><br /><math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="false"><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>D</mi><mo>⁢</mo><mi>M</mi></mrow></msub><mo>=</mo><mfrac><mrow><mi>S</mi><mo>⁢</mo><mrow><mo>(</mo><mi>n</mi><mo>-</mo><mi>m</mi><mo>)</mo></mrow></mrow><mrow><mn>2</mn><mo>⁢</mo><mrow><mo>(</mo><mi>m</mi><mo>+</mo><mi>n</mi><mo>)</mo></mrow></mrow></mfrac></mstyle></math><br />b) Khi n=7cm, m=3cm ta có:<br /><math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="false"><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi mathvariant="normal">D</mi><mo>⁢</mo><mi>M</mi></mrow></msub><mo>=</mo><mfrac><mrow><mn>7</mn><mo>-</mo><mn>3</mn></mrow><mrow><mn>2</mn><mo>⁢</mo><mrow><mo>(</mo><mn>7</mn><mo>+</mo><mn>3</mn><mo>)</mo></mrow></mrow></mfrac><mo>.</mo><mi>S</mi><mo>=</mo><mfrac><mi>S</mi><mn>5</mn></mfrac><mo>=</mo><mfrac><mrow><mn>20</mn><mo>.</mo><mi>S</mi></mrow><mn>100</mn></mfrac><mo>=</mo><mn>20</mn><mo>%</mo><mo>⁢</mo><mi>S</mi></mstyle></math><br />Vậy <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>D</mi><mo>⁢</mo><mi>M</mi></mrow></msub><mo>=</mo><mn>20</mn><mo>%</mo><mo>⁢</mo><msub><mi>S</mi><mrow><mi>A</mi><mo>⁢</mo><mi>B</mi><mo>⁢</mo><mi>C</mi></mrow></msub><mo>.</mo></math></p>
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