Cho <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-msubsup" id="MJXp-Span-3"><span class="MJXp-mn" id="MJXp-Span-4" style="margin-right: 0.05em;">9</span><span class="MJXp-mi MJXp-italic MJXp-script" id="MJXp-Span-5" style="vertical-align: 0.5em;">x</span></span><span class="MJXp-mo" id="MJXp-Span-6" style="margin-left: 0.267em; margin-right: 0.267em;">+</span><span class="MJXp-msubsup" id="MJXp-Span-7"><span class="MJXp-mn" id="MJXp-Span-8" style="margin-right: 0.05em;">9</span><span class="MJXp-mrow MJXp-script" id="MJXp-Span-9" style="vertical-align: 0.5em;"><span class="MJXp-mo" id="MJXp-Span-10">−</span><span class="MJXp-mi MJXp-italic" id="MJXp-Span-11">x</span></span></span><span class="MJXp-mo" id="MJXp-Span-12" style="margin-left: 0.333em; margin-right: 0.333em;">=</span><span class="MJXp-mn" id="MJXp-Span-13">14</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-msubsup"><span class="mjx-base"><span id="MJXc-Node-6" class="mjx-mn"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.396em; padding-bottom: 0.347em;">9</span></span></span><span class="mjx-sup" style="font-size: 70.7%; vertical-align: 0.591em; padding-left: 0px; padding-right: 0.071em;"><span id="MJXc-Node-7" class="mjx-mi" style=""><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.199em; padding-bottom: 0.298em;">x</span></span></span></span><span id="MJXc-Node-8" 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-9" class="mjx-msubsup MJXc-space2"><span class="mjx-base"><span id="MJXc-Node-10" class="mjx-mn"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.396em; padding-bottom: 0.347em;">9</span></span></span><span class="mjx-sup" style="font-size: 70.7%; vertical-align: 0.591em; padding-left: 0px; padding-right: 0.071em;"><span id="MJXc-Node-11" class="mjx-texatom" style=""><span id="MJXc-Node-12" class="mjx-mrow"><span id="MJXc-Node-13" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.298em; padding-bottom: 0.445em;">−</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: 0.298em;">x</span></span></span></span></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-mn MJXc-space3"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.396em; padding-bottom: 0.347em;">14</span></span></span></span></span></span></span><script type="math/tex" id="MathJax-Element-1">\Large 9^x+9^{-x}=14</script>. Khi đó biểu thức $\Large M=\dfrac{2+81^x+8

Cho 9x+9x=149x+9x=14. Khi đó biểu thức $\Large M=\dfrac{2+81^x+8

4.5/5

Tác giả: Thầy Tùng

Đăng ngày: 18 Aug 2022

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Câu hỏi:

Cho 9x+9x=149x+9x=14. Khi đó biểu thức M=2+81x+81x113x3xM=2+81x+81x113x3x có giá trị bằng 

Đáp án án đúng là: D

Lời giải chi tiết:

Chọn D

81x+81x=92x+92x=(9x+9x)22.9x.9x=(9x+9x)22=19481x+81x=92x+92x=(9x+9x)22.9x.9x=(9x+9x)22=194

(3x+3x)2=9x+9x+2=16(3x+3x)2=9x+9x+2=16 [3x+3x=43x+3x=4(vô lý) 3x+3x=4

Từ đó suy ra M=2+81x+81x113x3x=2+81x+81x11(3x+3x)=2+194114=28