Bài 2: Diện Tích Hình Chữ Nhật
Hướng dẫn giải Bài 6 (Trang 118 SGK Toán Hình học 8, Tập 1)
<p><strong class="content_question">Đề bài</strong></p>
<p>Diện tích hình chữ nhật thay đổi như thế nào nếu:</p>
<p>a) Chiều dài tăng <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"><mn>2</mn></math>"><span class="MJX_Assistive_MathML" role="presentation"><math xmlns="http://www.w3.org/1998/Math/MathML"><mn>2</mn></math></span></span> lần, chiều rộng không đổi?</p>
<p>b) Chiều dài và chiều rộng tăng 3 lần?</p>
<p>c) Chiều dài tăng 4 lần, chiều rộng giảm 4 lần?</p>
<p><strong class="content_detail">Lời giải chi tiết</strong></p>
<p>Giả sử hình chữ nhật ban đầu có chiều dài là <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></math>"><span id="MJXc-Node-20" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-21" class="mjx-mrow"><span id="MJXc-Node-22" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">a</span></span></span></span></span>, chiều rộng 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>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></p>
<p>Diện tích ban đầu của hình chữ nhật là: <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>S</mi><mo>=</mo><mi>a</mi><mo>.</mo><mi>b</mi></math>"><span id="MJXc-Node-26" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-27" class="mjx-mrow"><span id="MJXc-Node-28" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">S</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">a</span></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 MJXc-space1"><span class="mjx-char MJXc-TeX-math-I">b.</span></span></span></span></span></p>
<p>a) Nếu chiều dài tăng <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"><mn>2</mn></math>"><span class="MJX_Assistive_MathML" role="presentation"><math xmlns="http://www.w3.org/1998/Math/MathML"><mn>2</mn></math></span></span> lần, chiều rộng không đổi thì kích thước chiều dài và chiều rộng của hình chữ nhật mới là <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"><msup><mi>a</mi><mo>&#x2032;</mo></msup><mo>=</mo><mn>2</mn><mi>a</mi><mo>,</mo><msup><mi>b</mi><mo>&#x2032;</mo></msup><mo>=</mo><mi>b</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-msup"><span class="mjx-base"><span id="MJXc-Node-39" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">a</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 id="MJXc-Node-41" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-42" class="mjx-mn MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">2</span></span><span id="MJXc-Node-43" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">a</span></span><span id="MJXc-Node-44" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">,</span></span><span id="MJXc-Node-45" class="mjx-msup MJXc-space1"><span class="mjx-base"><span id="MJXc-Node-46" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">b</span></span></span><span class="mjx-sup"><span id="MJXc-Node-47" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span><span id="MJXc-Node-48" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-49" class="mjx-mi MJXc-space3"><span class="mjx-char MJXc-TeX-math-I">b</span></span></span></span></span>.</p>
<p>Diện tích hình chữ nhật mới là: <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"><msup><mi>S</mi><mo>&#x2032;</mo></msup><mo>=</mo><msup><mi>a</mi><mo>&#x2032;</mo></msup><mo>.</mo><msup><mi>b</mi><mo>&#x2032;</mo></msup><mo>=</mo><mn>2</mn><mi>a</mi><mo>.</mo><mi>b</mi></math>"><span id="MJXc-Node-50" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-51" class="mjx-mrow"><span id="MJXc-Node-52" class="mjx-msup"><span class="mjx-base"><span id="MJXc-Node-53" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">S</span></span></span><span class="mjx-sup"><span id="MJXc-Node-54" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span><span id="MJXc-Node-55" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-56" class="mjx-msup MJXc-space3"><span class="mjx-base"><span id="MJXc-Node-57" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">a</span></span></span><span class="mjx-sup"><span id="MJXc-Node-58" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span><span id="MJXc-Node-59" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">.</span></span><span id="MJXc-Node-60" class="mjx-msup MJXc-space1"><span class="mjx-base"><span id="MJXc-Node-61" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">b</span></span></span><span class="mjx-sup"><span id="MJXc-Node-62" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span><span id="MJXc-Node-63" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-64" class="mjx-mn MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">2</span></span><span id="MJXc-Node-65" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">a</span></span><span id="MJXc-Node-66" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">.</span></span><span id="MJXc-Node-67" class="mjx-mi MJXc-space1"><span class="mjx-char MJXc-TeX-math-I">b</span></span></span></span></span><span id="MathJax-Element-12-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"><mo>=</mo><mn>2</mn><mi>a</mi><mi>b</mi><mo>=</mo><mn>2</mn><mi>S</mi></math>"><span id="MJXc-Node-68" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-69" class="mjx-mrow"><span id="MJXc-Node-70" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-71" class="mjx-mn MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">2</span></span><span id="MJXc-Node-72" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">a</span></span><span id="MJXc-Node-73" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">b</span></span><span id="MJXc-Node-74" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-75" class="mjx-mn MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">2</span></span><span id="MJXc-Node-76" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">S</span></span></span></span><span class="MJX_Assistive_MathML" role="presentation"><math xmlns="http://www.w3.org/1998/Math/MathML"><mo>=</mo><mn>2</mn><mi>a</mi><mi>b</mi><mo>=</mo><mn>2</mn><mi>S</mi></math></span></span>.</p>
<p>Vậy diện tích hình chữ nhật mới gấp <span id="MathJax-Element-13-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"><mn>2</mn></math>"><span id="MJXc-Node-77" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-78" class="mjx-mrow"><span id="MJXc-Node-79" class="mjx-mn"><span class="mjx-char MJXc-TeX-main-R">2</span></span></span></span></span> lần diện tích hình chữ nhật ban đầu.</p>
<p>b) Nếu chiều dài và chiều rộng tăng <span id="MathJax-Element-14-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"><mn>3</mn></math>"><span id="MJXc-Node-80" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-81" class="mjx-mrow"><span id="MJXc-Node-82" class="mjx-mn"><span class="mjx-char MJXc-TeX-main-R">3</span></span></span></span></span> lần thì kích thước chiều dài và chiều rộng của hình chữ nhật mới là <span id="MathJax-Element-15-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"><msup><mi>a</mi><mo>&#x2032;</mo></msup><mo>=</mo><mn>3</mn><mi>a</mi><mo>,</mo><msup><mi>b</mi><mo>&#x2032;</mo></msup><mo>=</mo><mn>3</mn><mi>b</mi></math>"><span id="MJXc-Node-83" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-84" class="mjx-mrow"><span id="MJXc-Node-85" class="mjx-msup"><span class="mjx-base"><span id="MJXc-Node-86" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">a</span></span></span><span class="mjx-sup"><span id="MJXc-Node-87" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span><span id="MJXc-Node-88" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-89" class="mjx-mn MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">3</span></span><span id="MJXc-Node-90" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">a</span></span><span id="MJXc-Node-91" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">,</span></span><span id="MJXc-Node-92" class="mjx-msup MJXc-space1"><span class="mjx-base"><span id="MJXc-Node-93" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">b</span></span></span><span class="mjx-sup"><span id="MJXc-Node-94" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span><span id="MJXc-Node-95" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-96" class="mjx-mn MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">3</span></span><span id="MJXc-Node-97" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">b</span></span></span></span></span>.</p>
<p>Diện tích hình chữ nhật mới là: <span id="MathJax-Element-16-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"><msup><mi>S</mi><mo>&#x2032;</mo></msup><mo>=</mo><msup><mi>a</mi><mo>&#x2032;</mo></msup><mo>.</mo><msup><mi>b</mi><mo>&#x2032;</mo></msup><mo>=</mo><mn>3</mn><mi>a</mi><mn>.3</mn><mi>b</mi></math>"><span id="MJXc-Node-98" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-99" class="mjx-mrow"><span id="MJXc-Node-100" class="mjx-msup"><span class="mjx-base"><span id="MJXc-Node-101" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">S</span></span></span><span class="mjx-sup"><span id="MJXc-Node-102" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span><span id="MJXc-Node-103" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-104" class="mjx-msup MJXc-space3"><span class="mjx-base"><span id="MJXc-Node-105" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">a</span></span></span><span class="mjx-sup"><span id="MJXc-Node-106" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span><span id="MJXc-Node-107" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">.</span></span><span id="MJXc-Node-108" class="mjx-msup MJXc-space1"><span class="mjx-base"><span id="MJXc-Node-109" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">b</span></span></span><span class="mjx-sup"><span id="MJXc-Node-110" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span><span id="MJXc-Node-111" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-112" class="mjx-mn MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">3</span></span><span id="MJXc-Node-113" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">a</span></span><span id="MJXc-Node-114" class="mjx-mn"><span class="mjx-char MJXc-TeX-main-R">.3</span></span><span id="MJXc-Node-115" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">b</span></span></span></span></span><span id="MathJax-Element-17-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"><mo>=</mo><mn>9</mn><mi>a</mi><mi>b</mi><mo>=</mo><mn>9</mn><mi>S</mi></math>"><span id="MJXc-Node-116" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-117" class="mjx-mrow"><span id="MJXc-Node-118" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-119" class="mjx-mn MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">9</span></span><span id="MJXc-Node-120" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">a</span></span><span id="MJXc-Node-121" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">b</span></span><span id="MJXc-Node-122" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-123" class="mjx-mn MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">9</span></span><span id="MJXc-Node-124" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">S</span></span></span></span></span>.</p>
<p>Vậy diện tích hình chữ nhật mới gấp <span id="MathJax-Element-18-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"><mn>9</mn></math>"><span id="MJXc-Node-125" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-126" class="mjx-mrow"><span id="MJXc-Node-127" class="mjx-mn"><span class="mjx-char MJXc-TeX-main-R">9</span></span></span></span></span> lần diện tích hình chữ nhật ban đầu.</p>
<p>c) Nếu chiều dài tăng <span id="MathJax-Element-19-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"><mn>4</mn></math>"><span class="MJX_Assistive_MathML" role="presentation"><math xmlns="http://www.w3.org/1998/Math/MathML"><mn>4</mn></math></span></span> lần, chiều rộng giảm <span id="MathJax-Element-20-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"><mn>4</mn></math>"><span id="MJXc-Node-131" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-132" class="mjx-mrow"><span id="MJXc-Node-133" class="mjx-mn"><span class="mjx-char MJXc-TeX-main-R">4</span></span></span></span></span> lần thì kích thước chiều dài và chiều rộng của hình chữ nhật mới là <span id="MathJax-Element-21-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"><msup><mi>a</mi><mo>&#x2032;</mo></msup><mo>=</mo><mn>4</mn><mi>a</mi><mo>,</mo></math>"><span id="MJXc-Node-134" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-135" class="mjx-mrow"><span id="MJXc-Node-136" class="mjx-msup"><span class="mjx-base"><span id="MJXc-Node-137" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">a</span></span></span><span class="mjx-sup"><span id="MJXc-Node-138" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">′</span></span></span></span><span id="MJXc-Node-139" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">=</span></span><span id="MJXc-Node-140" class="mjx-mn MJXc-space3"><span class="mjx-char MJXc-TeX-main-R">4</span></span><span id="MJXc-Node-141" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I">a</span></span><span id="MJXc-Node-142" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R">,</span></span></span></span></span></p>
<p><math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>b</mi><mo>'</mo></msup><mo>=</mo><mfrac><mi>b</mi><mn>4</mn></mfrac><mo>⁢</mo><mo>.</mo></math><br />Diện tích hình chữ nhật mới là $<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>S</mi><mo>'</mo></msup><mo>=</mo><msup><mi>a</mi><mo>'</mo></msup><mo>⋅</mo><msup><mi>b</mi><mo>'</mo></msup><mo>=</mo><mn>4</mn><mo>⁢</mo><mi>a</mi><mo>⋅</mo><mfrac><mi>b</mi><mn>4</mn></mfrac><mo>=</mo><mi>a</mi><mo>⁢</mo><mi>b</mi><mo>=</mo><mi>S</mi><mo>.</mo></math><br />Vậy diện tích hình chữ nhật không đổi.<br /><span id="MathJax-Element-21-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"><msup><mi>a</mi><mo>&#x2032;</mo></msup><mo>=</mo><mn>4</mn><mi>a</mi><mo>,</mo></math>"><span id="MJXc-Node-134" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-135" class="mjx-mrow"><span id="MJXc-Node-142" class="mjx-mo"></span></span></span></span></p>
<p> </p>
<p><br /><br /></p>
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