Cho n là số dương 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-mn" id="MJXp-Span-3">5</span><span class="MJXp-msubsup" id="MJXp-Span-4"><span class="MJXp-mi MJXp-italic" id="MJXp-Span-5" style="margin-right: 0.05em;">C</span><span class="MJXp-script-box" style="height: 1.86em; vertical-align: -0.64em;"><span class=" MJXp-script"><span><span style="margin-bottom: -0.25em;"><span class="MJXp-mrow" id="MJXp-Span-8"><span class="MJXp-mi MJXp-italic" id="MJXp-Span-9">n</span><span class="MJXp-mo" id="MJXp-Span-10">−</span><span class="MJXp-mn" id="MJXp-Span-11">1</span></span></span></span></span><span class=" MJXp-script"><span><span style="margin-top: -0.85em;"><span class="MJXp-mrow" id="MJXp-Span-6"><span class="MJXp-mi MJXp-italic" id="MJXp-Span-7">n</span></span></span></span></span></span></span><span class="MJXp-mo" id="MJXp-Span-12" style="margin-left: 0.333em; margin-right: 0.333em;">=</span><span class="MJXp-msubsup" id="MJXp-Span-13"><span class="MJXp-mi MJXp-italic" id="MJXp-Span-14" style="margin-right: 0.05em;">C</span><span class="MJXp-script-box" style="height: 1.86em; vertical-align: -0.64em;"><span class=" MJXp-script"><span><span style="margin-bottom: -0.25em;"><span class="MJXp-mrow" id="MJXp-Span-17"><span class="MJXp-mn" id="MJXp-Span-18">3</span></span></span></span></span><span class=" MJXp-script"><span><span style="margin-top: -0.85em;"><span class="MJXp-mrow" id="MJXp-Span-15"><span class="MJXp-mi MJXp-italic" id="MJXp-Span-16">n</span></span></span></span></span></span></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-mn"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.396em; padding-bottom: 0.347em;">5</span></span><span id="MJXc-Node-6" class="mjx-msubsup"><span class="mjx-base" style="margin-right: -0.045em;"><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><span class="mjx-stack" style="vertical-align: -0.157em;"><span class="mjx-sup" style="font-size: 70.7%; padding-bottom: 0.255em; padding-left: 0.153em; 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-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.199em; padding-bottom: 0.298em;">n</span></span><span id="MJXc-Node-14" 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-15" class="mjx-mn"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.396em; padding-bottom: 0.347em;">1</span></span></span></span></span><span class="mjx-sub" style="font-size: 70.7%; padding-right: 0.071em;"><span id="MJXc-Node-8" class="mjx-texatom" style=""><span id="MJXc-Node-9" class="mjx-mrow"><span id="MJXc-Node-10" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.199em; padding-bottom: 0.298em;">n</span></span></span></span></span></span></span><span id="MJXc-Node-16" 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-17" class="mjx-msubsup MJXc-space3"><span class="mjx-base" style="margin-right: -0.045em;"><span id="MJXc-Node-18" 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><span class="mjx-stack" style="vertical-align: -0.157em;"><span class="mjx-sup" style="font-size: 70.7%; padding-bottom: 0.255em; padding-left: 0.153em; padding-right: 0.071em;"><span id="MJXc-Node-22" class="mjx-texatom" style=""><span id="MJXc-Node-23" class="mjx-mrow"><span id="MJXc-Node-24" class="mjx-mn"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.396em; padding-bottom: 0.347em;">3</span></span></span></span></span><span class="mjx-sub" style="font-size: 70.7%; padding-right: 0.071em;"><span id="MJXc-Node-19" class="mjx-texatom" style=""><span id="MJXc-Node-20" class="mjx-mrow"><span id="MJXc-Node-21" class="mjx-mi"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.199em; padding-bottom: 0.298em;">n</span></span></span></span></span></span></span></span></span></span></span></span><script type="math/tex" id="MathJax-Element-1">\Large 5 C_{n}^{n-1}=C_{n}^{3}</script>. Số hạng c

Cho n là số dương thỏa mãn 5Cn1n=C3n5Cn1n=C3n. Số hạng c

4.1/5

Tác giả: Thầy Tùng

Đăng ngày: 18 Aug 2022

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Câu hỏi:

Cho n là số dương thỏa mãn 5Cn1n=C3n5Cn1n=C3n. Số hạng chứa x5x5 trong khai triển nhị thức Newton P=(nx2141x)nP=(nx2141x)n với x0x0 là?

Đáp án án đúng là: C

Lời giải chi tiết:

Chọn C

Điều kiện nN,n3nN,n3

Ta có: 5Cn1n=C3n5.n!1!(n1)!=n!3!(n3)!5Cn1n=C3n5.n!1!(n1)!=n!3!(n3)!5(n3)!(n2)(n1)=16(n3)!5(n3)!(n2)(n1)=16(n3)!

n23n28=0[n=7 (thỏa mãn) n=4 (loại) 

Với n=7 ta có P=(x221x)7

P=(x221x)7=7k=0Ck7(x22)7k(1x)k=7k=0Ck7127k(1)7kx143k

Số hạng chứa x5 tương ứng với 143k=5k=3

Vậy số hạng chứa x5 trong khai triển là C47124(1)3=3516x5