Cho tứ diện <span class="MathJax_Preview" style="color: inherit;"><span class="MJXp-math" id="MJXp-Span-1"><span class="MJXp-mstyle" id="MJXp-Span-2"><span class="MJXp-mi MJXp-italic" id="MJXp-Span-3">A</span><span class="MJXp-mi MJXp-italic" id="MJXp-Span-4">B</span><span class="MJXp-mi MJXp-italic" id="MJXp-Span-5">C</span><span class="MJXp-mi MJXp-italic" id="MJXp-Span-6">D</span></span></span></span><span id="MathJax-Element-1-Frame" class="mjx-chtml MathJax_CHTML MJXc-processed" tabindex="0" style="font-size: 127%;"><span id="MJXc-Node-1" class="mjx-math"><span id="MJXc-Node-2" class="mjx-mrow"><span id="MJXc-Node-3" class="mjx-mstyle"><span id="MJXc-Node-4" class="mjx-mrow" style="font-size: 120%;"><span id="MJXc-Node-5" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.495em; padding-bottom: 0.298em;">A</span></span><span id="MJXc-Node-6" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.445em; padding-bottom: 0.298em;">B</span></span><span id="MJXc-Node-7" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.495em; padding-bottom: 0.298em; padding-right: 0.045em;">C</span></span><span id="MJXc-Node-8" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.445em; padding-bottom: 0.298em;">D</span></span></span></span></span></span></span><script type="math/tex" id="MathJax-Element-1">\large ABCD</script> có các cạnh <span class="MathJax_Preview" style="color: inherit;"><span class="MJXp-math" id="MJXp-Span-7"><span class="MJXp-mstyle" id="MJXp-Span-8"><span class="MJXp-mi MJXp-italic" id="MJXp-Span-9">A</span><span class="MJXp-mi MJXp-italic" id="MJXp-Span-10">B</span><span class="MJXp-mo" id="MJXp-Span-11" style="margin-left: 0.333em; margin-right: 0.333em;">=</span><span class="MJXp-mi MJXp-italic" id="MJXp-Span-12">B</span><span class="MJXp-mi MJXp-italic" id="MJXp-Span-13">C</span><span class="MJXp-mo" id="MJXp-Span-14" style="margin-left: 0.333em; margin-right: 0.333em;">=</span><span class="MJXp-mi MJXp-italic" id="MJXp-Span-15">C</span><span class="MJXp-mi MJXp-italic" id="MJXp-Span-16">D</span><span class="MJXp-mo" id="MJXp-Span-17" style="margin-left: 0.333em; margin-right: 0.333em;">=</span><span class="MJXp-mi MJXp-italic" id="MJXp-Span-18">D</span><span class="MJXp-mi MJXp-italic" id="MJXp-Span-19">A</span><span class="MJXp-mo" id="MJXp-Span-20" style="margin-left: 0.333em; margin-right: 0.333em;">=</span><span class="MJXp-mn" id="MJXp-Span-21">1</span></span></span></span><span id="MathJax-Element-2-Frame" class="mjx-chtml MathJax_CHTML MJXc-processed" tabindex="0" style="font-size: 127%;"><span id="MJXc-Node-9" class="mjx-math"><span id="MJXc-Node-10" class="mjx-mrow"><span id="MJXc-Node-11" class="mjx-mstyle"><span id="MJXc-Node-12" class="mjx-mrow" style="font-size: 120%;"><span id="MJXc-Node-13" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.495em; padding-bottom: 0.298em;">A</span></span><span id="MJXc-Node-14" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.445em; padding-bottom: 0.298em;">B</span></span><span id="MJXc-Node-15" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.101em; padding-bottom: 0.298em;">=</span></span><span id="MJXc-Node-16" class="mjx-mi MJXc-space3"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.445em; padding-bottom: 0.298em;">B</span></span><span id="MJXc-Node-17" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.495em; padding-bottom: 0.298em; padding-right: 0.045em;">C</span></span><span id="MJXc-Node-18" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.101em; padding-bottom: 0.298em;">=</span></span><span id="MJXc-Node-19" class="mjx-mi MJXc-space3"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.495em; padding-bottom: 0.298em; padding-right: 0.045em;">C</span></span><span id="MJXc-Node-20" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.445em; padding-bottom: 0.298em;">D</span></span><span id="MJXc-Node-21" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.101em; padding-bottom: 0.298em;">=</span></span><span id="MJXc-Node-22" class="mjx-mi MJXc-space3"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.445em; padding-bottom: 0.298em;">D</span></span><span id="MJXc-Node-23" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.495em; padding-bottom: 0.298em;">A</span></span><span id="MJXc-Node-24" class="mjx-mo MJXc-space3"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.101em; padding-bottom: 0.298em;">=</span></span><span id="MJXc-Node-25" class="mjx-mn MJXc-space3"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.396em; padding-bottom: 0.347em;">1</span></span></span></span></span></span></span><script type="math/tex" id="MathJax-Element-2">\large AB=BC=CD=DA=1</script> và $\larg

Cho tứ diện ABCDABCD có các cạnh AB=BC=CD=DA=1AB=BC=CD=DA=1 và $\larg

4.5/5

Tác giả: Thầy Tùng

Đăng ngày: 18 Aug 2022

Lưu về Facebook:
Hình minh họa Cho tứ diện $\large ABCD$ có các cạnh $\large AB=BC=CD=DA=1$ và $\larg

Câu hỏi:

Cho tứ diện ABCDABCD có các cạnh AB=BC=CD=DA=1AB=BC=CD=DA=1AC,BDAC,BD thay đổi. Thể tích ABCDABCD đạt giá trị lớn nhất bằng:

Đáp án án đúng là: B

Lời giải chi tiết:

Hình đáp án 1. Cho tứ diện $\large ABCD$ có các cạnh $\large AB=BC=CD=DA=1$ và $\larg

Đặt AC=x,BD=y(x,y>0)AC=x,BD=y(x,y>0)

Gọi E,FE,F lần lượt là trung điểm AC,BDAC,BD

ABC=△ADC(c.c.c)DE=BEEFBDABC=ADC(c.c.c)DE=BEEFBD. Chứng minh tương tự EFACEFAC

Suy ra EFEF là đoạn vuông góc chung của AC,BDAC,BD

Ta có {ACEFACBE AC(BED)

VABCD=2VABDE=213AE.SBED=23AE.EF.BF=23x2y2EF (1)

Trong BEF:EF2=BE2BF2=1x24y24.

Ta có:

V2ABCD=1144x2y2(4x2y2)1144(x2+y2+(4x2y2)3)3=4243maxVABCD=2327

Dấu "=" xảy ra khi x2=y2=4x2y2x=y=2327

Đáp án B