Cho x, y là các số thực thỏa mãn <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-mo" id="MJXp-Span-3" style="margin-left: 0em; margin-right: 0em;">(</span><span class="MJXp-mi MJXp-italic" id="MJXp-Span-4">x</span><span class="MJXp-mo" id="MJXp-Span-5" style="margin-left: 0.267em; margin-right: 0.267em;">−</span><span class="MJXp-mn" id="MJXp-Span-6">3</span><span class="MJXp-msubsup" id="MJXp-Span-7"><span class="MJXp-mo" id="MJXp-Span-8" style="margin-left: 0em; margin-right: 0.05em;">)</span><span class="MJXp-mn MJXp-script" id="MJXp-Span-9" style="vertical-align: 0.5em;">2</span></span><span class="MJXp-mo" id="MJXp-Span-10" style="margin-left: 0.267em; margin-right: 0.267em;">+</span><span class="MJXp-mo" id="MJXp-Span-11" style="margin-left: 0em; margin-right: 0em;">(</span><span class="MJXp-mi MJXp-italic" id="MJXp-Span-12">y</span><span class="MJXp-mo" id="MJXp-Span-13" style="margin-left: 0.267em; margin-right: 0.267em;">−</span><span class="MJXp-mn" id="MJXp-Span-14">1</span><span class="MJXp-msubsup" id="MJXp-Span-15"><span class="MJXp-mo" id="MJXp-Span-16" style="margin-left: 0em; margin-right: 0.05em;">)</span><span class="MJXp-mn MJXp-script" id="MJXp-Span-17" style="vertical-align: 0.5em;">2</span></span><span class="MJXp-mo" id="MJXp-Span-18" style="margin-left: 0.333em; margin-right: 0.333em;">=</span><span class="MJXp-mn" id="MJXp-Span-19">5</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: 144%;"><span id="MJXc-Node-5" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.445em; padding-bottom: 0.593em;">(</span></span><span id="MJXc-Node-6" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.199em; padding-bottom: 0.298em;">x</span></span><span id="MJXc-Node-7" class="mjx-mo MJXc-space2"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.298em; padding-bottom: 0.445em;">−</span></span><span id="MJXc-Node-8" class="mjx-mn MJXc-space2"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.396em; padding-bottom: 0.347em;">3</span></span><span id="MJXc-Node-9" class="mjx-msubsup"><span class="mjx-base"><span id="MJXc-Node-10" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.445em; padding-bottom: 0.593em;">)</span></span></span><span class="mjx-sup" style="font-size: 70.7%; vertical-align: 0.513em; padding-left: 0px; padding-right: 0.071em;"><span id="MJXc-Node-11" class="mjx-mn" style=""><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.396em; padding-bottom: 0.347em;">2</span></span></span></span><span id="MJXc-Node-12" class="mjx-mo MJXc-space2"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.298em; padding-bottom: 0.445em;">+</span></span><span id="MJXc-Node-13" class="mjx-mo MJXc-space2"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.445em; padding-bottom: 0.593em;">(</span></span><span id="MJXc-Node-14" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.199em; padding-bottom: 0.495em; padding-right: 0.006em;">y</span></span><span id="MJXc-Node-15" class="mjx-mo MJXc-space2"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.298em; padding-bottom: 0.445em;">−</span></span><span id="MJXc-Node-16" class="mjx-mn MJXc-space2"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.396em; padding-bottom: 0.347em;">1</span></span><span id="MJXc-Node-17" class="mjx-msubsup"><span class="mjx-base"><span id="MJXc-Node-18" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.445em; padding-bottom: 0.593em;">)</span></span></span><span class="mjx-sup" style="font-size: 70.7%; vertical-align: 0.513em; padding-left: 0px; padding-right: 0.071em;"><span id="MJXc-Node-19" class="mjx-mn" style=""><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.396em; padding-bottom: 0.347em;">2</span></span></span></span><span id="MJXc-Node-20" 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-21" class="mjx-mn MJXc-space3"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.396em; padding-bottom: 0.347em;">5</span></span></span></span></span></span></span><script type="math/tex" id="MathJax-Element-1">\Large (x-3)^2+(y-1)^2=5</script>. Giá trị n

Cho x, y là các số thực thỏa mãn (x3)2+(y1)2=5(x3)2+(y1)2=5. Giá trị n

4.1/5

Tác giả: Thầy Tùng

Đăng ngày: 18 Aug 2022

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Câu hỏi:

Cho x, y là các số thực thỏa mãn (x3)2+(y1)2=5(x3)2+(y1)2=5. Giá trị nhỏ nhất của biểu thức P=3y2+4xy+7x+4y1x+2y+1P=3y2+4xy+7x+4y1x+2y+1 là: 

Đáp án án đúng là: D

Lời giải chi tiết:

Chọn D

Từ giả thiết ta có: x26x+9+y22y+1=56x+2y=x2+y2+5x26x+9+y22y+1=56x+2y=x2+y2+5

P=3y2+4xy+7x+4y1x+2y+1=3y2+4xy+6x+2y+x+2y1x+2y+1P=3y2+4xy+7x+4y1x+2y+1=3y2+4xy+6x+2y+x+2y1x+2y+1

P=x2+4y2+4xy+x+2y+4x+2y+1=x(x+2y+1)+2y(x+2y+1)+4x+2y+1=x+2y+4x+2y+1

Đặt t=x+2yP=t+4t+1

Áp dụng BĐT Bunhiacopxki ta có: [(x3)+2(y1)]2(12+22)[(x3)2+(y1)2]=25

5(x3)+2(y1)55x+2y550t10

Áp dụng BĐT Cauchy ta có: t+1+4t+14P3. Dấu bằng xảy ra t+1=4t+1t=1

Khi đó: [x+2y=1(x3)2+(y1)2=5 [{x=1y=0{x=175y=65