Bài 2: Định Lí Đảo Và Hệ Quả Của Định Lí Ta-Lét
Hướng dẫn giải Bài 12 (Trang 64 SGK Toán Hình học 8, Tập 2)
<p><strong class="content_question">Đề bài</strong></p>
<p>Có thể đo dược chiều rộng của một khúc sông mà không cần phải sang bờ bên kia hay không?</p>
<p>Người ta tiến hành đo đạc các yếu tố hình học cần thiết để tình chiều rộng của khúc sông mà không cần phải sang bờ bên kia(h18). Nhìn hình vẽ, Hãy mô tả những công việc cần làm và tính khoảng cách <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><mo>=</mo><mi>x</mi></math>"><span class="MJX_Assistive_MathML" role="presentation"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>A</mi><mi>B</mi><mo>=</mo><mi>x</mi></math></span></span> theo <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>B</mi><mi>C</mi><mo>=</mo><mi>a</mi><mo>,</mo><msup><mi>B</mi><mo>&#x2032;</mo></msup><msup><mi>C</mi><mo>&#x2032;</mo></msup><mo>=</mo><msup><mi>a</mi><mo>&#x2032;</mo></msup><mo>,</mo><mi>B</mi><msup><mi>B</mi><mo>&#x2032;</mo></msup><mo>=</mo><mi>h</mi></math>"><span id="MJXc-Node-7" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-8" class="mjx-mrow"><span id="MJXc-Node-9" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">B</span></span><span id="MJXc-Node-10" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">C</span></span><span id="MJXc-Node-11" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-12" class="mjx-mi MJXc-space3"><span class="mjx-char MJXc-TeX-math-I">a</span></span><span id="MJXc-Node-13" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">,</span></span><span id="MJXc-Node-14" class="mjx-msup MJXc-space1"><span class="mjx-base"><span id="MJXc-Node-15" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">B</span></span></span><span class="mjx-sup"><span id="MJXc-Node-16" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span><span id="MJXc-Node-17" class="mjx-msup"><span class="mjx-base"><span id="MJXc-Node-18" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">C</span></span></span><span class="mjx-sup"><span id="MJXc-Node-19" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span><span id="MJXc-Node-20" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-21" class="mjx-msup MJXc-space3"><span class="mjx-base"><span id="MJXc-Node-22" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">a</span></span></span><span class="mjx-sup"><span id="MJXc-Node-23" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span><span id="MJXc-Node-24" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">,</span></span><span id="MJXc-Node-25" class="mjx-mi MJXc-space1"><span class="mjx-char MJXc-TeX-math-I">B</span></span><span id="MJXc-Node-26" class="mjx-msup"><span class="mjx-base"><span id="MJXc-Node-27" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">B</span></span></span><span class="mjx-sup"><span id="MJXc-Node-28" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span><span id="MJXc-Node-29" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-30" class="mjx-mi MJXc-space3"><span class="mjx-char MJXc-TeX-math-I">h</span></span></span></span></span>.</p>
<p><img src="https://img.loigiaihay.com/picture/2018/0718/b12-trang-64-sgk-toan-8-t2-c2.jpg" /></p>
<p><strong class="content_detail">Lời giải chi tiết</strong></p>
<p> </p>
<p>Mô tả cách làm:</p>
<p>* Chọn một điểm A cố định bên mép bờ sông bên kia (chẳng hạn như là một thân cây), đặt hai điểm B và B' thẳng hàng với A, điểm B sát mép bờ còn lại và <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>B</mi></math>"><span id="MJXc-Node-31" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-32" class="mjx-mrow"><span id="MJXc-Node-33" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">A</span></span><span id="MJXc-Node-34" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">B</span></span></span></span></span> chính là khoảng cách cần đo.</p>
<p>* Trên hai đường thẳng vuông góc với <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: 400; font-size: 16.94px; letter-spacing: normal; overflow-wrap: normal; word-spacing: 0px; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; color: #000000; font-family: OpenSans, Tahoma, Helvetica, sans-serif; font-variant-ligatures: normal; font-variant-caps: normal; orphans: 2; widows: 2; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>A</mi><msup><mi>B</mi><mo>&#x2032;</mo></msup></math>"><span id="MJXc-Node-35" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-36" class="mjx-mrow"><span id="MJXc-Node-37" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">A</span></span><span id="MJXc-Node-38" class="mjx-msup"><span class="mjx-base"><span id="MJXc-Node-39" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">B</span></span></span><span class="mjx-sup"><span id="MJXc-Node-40" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span></span></span></span> tại <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: 400; font-size: 16.94px; letter-spacing: normal; overflow-wrap: normal; word-spacing: 0px; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; color: #000000; font-family: OpenSans, Tahoma, Helvetica, sans-serif; font-variant-ligatures: normal; font-variant-caps: normal; orphans: 2; widows: 2; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>B</mi></math>"><span class="MJX_Assistive_MathML" role="presentation"><math xmlns="http://www.w3.org/1998/Math/MathML"><mi>B</mi></math></span></span> và <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: 400; font-size: 16.94px; letter-spacing: normal; overflow-wrap: normal; word-spacing: 0px; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; color: #000000; font-family: OpenSans, Tahoma, Helvetica, sans-serif; font-variant-ligatures: normal; font-variant-caps: normal; orphans: 2; widows: 2; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>B</mi><mo>&#x2032;</mo></msup></math>"><span class="MJX_Assistive_MathML" role="presentation"><math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>B</mi><mo>′</mo></msup></math></span></span> lấy C và <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: 400; font-size: 16.94px; letter-spacing: normal; overflow-wrap: normal; word-spacing: 0px; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; color: #000000; font-family: OpenSans, Tahoma, Helvetica, sans-serif; font-variant-ligatures: normal; font-variant-caps: normal; orphans: 2; widows: 2; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>C</mi><mo>&#x2032;</mo></msup></math>"><span class="MJX_Assistive_MathML" role="presentation"><math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>C</mi><mo>′</mo></msup></math></span></span> sao cho <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: 400; font-size: 16.94px; letter-spacing: normal; overflow-wrap: normal; word-spacing: 0px; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; color: #000000; font-family: OpenSans, Tahoma, Helvetica, sans-serif; font-variant-ligatures: normal; font-variant-caps: normal; orphans: 2; widows: 2; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>A</mi><mo>,</mo><mi>C</mi><mo>,</mo><msup><mi>C</mi><mo>&#x2032;</mo></msup></math>"><span id="MJXc-Node-57" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-58" class="mjx-mrow"><span id="MJXc-Node-59" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">A</span></span><span id="MJXc-Node-60" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">,</span></span><span id="MJXc-Node-61" class="mjx-mi MJXc-space1"><span class="mjx-char MJXc-TeX-math-I">C</span></span><span id="MJXc-Node-62" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">,</span></span><span id="MJXc-Node-63" class="mjx-msup MJXc-space1"><span class="mjx-base"><span id="MJXc-Node-64" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">C</span></span></span><span class="mjx-sup"><span id="MJXc-Node-65" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span></span></span></span> thẳng hàng.<br />* Đo độ dài các đoạn <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: 400; font-size: 16.94px; letter-spacing: normal; overflow-wrap: normal; word-spacing: 0px; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; color: #000000; font-family: OpenSans, Tahoma, Helvetica, sans-serif; font-variant-ligatures: normal; font-variant-caps: normal; orphans: 2; widows: 2; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>B</mi><msup><mi>B</mi><mo>&#x2032;</mo></msup><mo>=</mo><mi>h</mi><mo>,</mo><mi>B</mi><mi>C</mi><mo>=</mo><mi>a</mi><mo>,</mo><msup><mi>B</mi><mo>&#x2032;</mo></msup><msup><mi>C</mi><mo>&#x2032;</mo></msup><mo>=</mo><msup><mi>a</mi><mo>&#x2032;</mo></msup></math>"><span id="MJXc-Node-66" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-67" class="mjx-mrow"><span id="MJXc-Node-68" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">B</span></span><span id="MJXc-Node-69" class="mjx-msup"><span class="mjx-base"><span id="MJXc-Node-70" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">B</span></span></span><span class="mjx-sup"><span id="MJXc-Node-71" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span><span id="MJXc-Node-72" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-73" class="mjx-mi MJXc-space3"><span class="mjx-char MJXc-TeX-math-I">h</span></span><span id="MJXc-Node-74" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">,</span></span><span id="MJXc-Node-75" class="mjx-mi MJXc-space1"><span class="mjx-char MJXc-TeX-math-I">B</span></span><span id="MJXc-Node-76" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">C</span></span><span id="MJXc-Node-77" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-78" class="mjx-mi MJXc-space3"><span class="mjx-char MJXc-TeX-math-I">a</span></span><span id="MJXc-Node-79" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">,</span></span><span id="MJXc-Node-80" class="mjx-msup MJXc-space1"><span class="mjx-base"><span id="MJXc-Node-81" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">B</span></span></span><span class="mjx-sup"><span id="MJXc-Node-82" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span><span id="MJXc-Node-83" class="mjx-msup"><span class="mjx-base"><span id="MJXc-Node-84" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">C</span></span></span><span class="mjx-sup"><span id="MJXc-Node-85" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span><span id="MJXc-Node-86" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-87" class="mjx-msup MJXc-space3"><span class="mjx-base"><span id="MJXc-Node-88" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">a</span></span></span><span class="mjx-sup"><span id="MJXc-Node-89" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span></span></span></span>. Từ đó ta sẽ tính được đoạn <span id="MathJax-Element-11-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>x</mi><mo>.</mo></math>"><span id="MJXc-Node-90" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-91" class="mjx-mrow"><span id="MJXc-Node-92" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">A</span></span><span id="MJXc-Node-93" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">B</span></span><span id="MJXc-Node-94" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=x.</span></span></span></span></span></p>
<p><span id="MathJax-Element-11-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>x</mi><mo>.</mo></math>"><span id="MJXc-Node-90" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-91" class="mjx-mrow"><span id="MJXc-Node-95" class="mjx-mi MJXc-space3"><span class="mjx-char MJXc-TeX-math-I"><strong>Giải</strong></span></span></span></span></span></p>
<p>Ta có: <math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="false"><mi>B</mi><mo>⁢</mo><mi>C</mi><mo>⟂</mo><mi>A</mi><mo>⁢</mo><msup><mi>B</mi><mo>'</mo></msup></mstyle></math> và <math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="false"><msup><mi>B</mi><mo>'</mo></msup><msup><mi>C</mi><mo>'</mo></msup><mo>⟂</mo><mi>A</mi><msup><mi>B</mi><mo>'</mo></msup><mo>⇒</mo><mi>B</mi><mi>C</mi><mo>/</mo><mo>/</mo><msup><mi>B</mi><mo>'</mo></msup><msup><mi>C</mi><mo>'</mo></msup></mstyle></math><br />Xét <math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="false"><mi mathvariant="normal">Δ</mi><mo>⁢</mo><mi>A</mi><mo>⁢</mo><msup><mi>B</mi><mo>'</mo></msup><mo>⁢</mo><msup><mi>C</mi><mo>'</mo></msup></mstyle></math> có <math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="false"><mi>B</mi><mi>C</mi><mo>/</mo><mo>/</mo><msup><mi>B</mi><mo>'</mo></msup><msup><mi>C</mi><mo>'</mo></msup><mrow><mo>(</mo><mi>B</mi><mo>∈</mo><mi>A</mi><msup><mi>B</mi><mo>'</mo></msup><mo>,</mo><mi>C</mi><mo>∈</mo><mi>A</mi><msup><mi>C</mi><mo>'</mo></msup><mo>)</mo></mrow></mstyle></math><br /><math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="false"><mo>⇒</mo><mfrac><mrow><mi>A</mi><mo>⁢</mo><mi>B</mi></mrow><mrow><mi>A</mi><mo>⁢</mo><msup><mi>B</mi><mo>'</mo></msup></mrow></mfrac><mo>=</mo><mfrac><mrow><mi>B</mi><mo>⁢</mo><mi>C</mi></mrow><mrow><mi>B</mi><mo>⁢</mo><msup><mi>C</mi><mo>'</mo></msup></mrow></mfrac></mstyle></math> (hệ quả định lý Talet) mà <math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="false"><mi>A</mi><mo>⁢</mo><msup><mi>B</mi><mo>'</mo></msup><mo>=</mo><mi>x</mi><mo>+</mo><mi>h</mi></mstyle></math> nên <math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="false"><mfrac><mi>x</mi><mrow><mi>x</mi><mo>+</mo><mi>h</mi></mrow></mfrac><mo>=</mo><mfrac><mi>a</mi><msup><mi>a</mi><mo>'</mo></msup></mfrac></mstyle></math><br /><math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="false"><mo>⇔</mo><msup><mi>a</mi><mo>'</mo></msup><mo>⁢</mo><mi>x</mi><mo>=</mo><mi>a</mi><mo>⁢</mo><mi>x</mi><mo>+</mo><mi>a</mi><mo>⁢</mo><mi>h</mi></mstyle></math><br /><math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="false"><mo>⇔</mo><msup><mi>a</mi><mo>'</mo></msup><mo>⁢</mo><mi>x</mi><mo>-</mo><mi>a</mi><mo>⁢</mo><mi>x</mi><mo>=</mo><mi>a</mi><mo>⁢</mo><mi>h</mi></mstyle></math><br /><math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="false"><mo>⇔</mo><mi>x</mi><mo>⁢</mo><mrow><mo>(</mo><msup><mi>a</mi><mo>'</mo></msup><mo>-</mo><mi>a</mi><mo>)</mo></mrow><mo>=</mo><mi>a</mi><mo>⁢</mo><mi>h</mi></mstyle></math><br /><math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="false"><mo>⇒</mo><mi>x</mi><mo>=</mo><mfrac><mrow><mi>a</mi><mo>⁢</mo><mi>h</mi></mrow><mrow><msup><mi>a</mi><mo>'</mo></msup><mo>-</mo><mi>a</mi></mrow></mfrac></mstyle></math><br />Vậy khoảng cách AB bằng <math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="false"><mfrac><mrow><mi>a</mi><mo>⁢</mo><mi>h</mi></mrow><mrow><msup><mi>a</mi><mo>'</mo></msup><mo>-</mo><mi>a</mi></mrow></mfrac></mstyle></math><br /><br /><br /></p>
<p> </p>
<p> </p>
<p> </p>
<p><br /><br /></p>
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