Cho <span class="MathJax_Preview" style="color: inherit; display: none;"></span><span id="MathJax-Element-1-Frame" class="mjx-chtml MathJax_CHTML" tabindex="0" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle mathsize="1.44em"><msub><mi>log</mi><mrow class="MJX-TeXAtom-ORD"><mn>14</mn></mrow></msub><mo>&#x2061;</mo><mn>7</mn><mo>=</mo><mi>a</mi><mo>;</mo><msub><mi>log</mi><mrow class="MJX-TeXAtom-ORD"><mn>14</mn></mrow></msub><mo>&#x2061;</mo><mn>5</mn><mo>=</mo><mi>b</mi></mstyle></math>" role="presentation" style="font-size: 127%; position: relative;"><span id="MJXc-Node-1" class="mjx-math" aria-hidden="true"><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-mi"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.396em; padding-bottom: 0.544em;">log</span></span></span><span class="mjx-sub" style="font-size: 70.7%; vertical-align: -0.377em; padding-right: 0.071em;"><span id="MJXc-Node-7" class="mjx-texatom" style=""><span id="MJXc-Node-8" class="mjx-mrow"><span id="MJXc-Node-9" class="mjx-mn"><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 id="MJXc-Node-10" class="mjx-mo"><span class="mjx-char"></span></span><span id="MJXc-Node-11" class="mjx-mn MJXc-space1"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.396em; padding-bottom: 0.347em;">7</span></span><span id="MJXc-Node-12" 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-13" class="mjx-mi MJXc-space3"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.199em; padding-bottom: 0.298em;">a</span></span><span id="MJXc-Node-14" class="mjx-mo"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.15em; padding-bottom: 0.544em;">;</span></span><span id="MJXc-Node-15" class="mjx-msubsup MJXc-space1"><span class="mjx-base"><span id="MJXc-Node-16" class="mjx-mi"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.396em; padding-bottom: 0.544em;">log</span></span></span><span class="mjx-sub" style="font-size: 70.7%; vertical-align: -0.377em; padding-right: 0.071em;"><span id="MJXc-Node-17" class="mjx-texatom" style=""><span id="MJXc-Node-18" class="mjx-mrow"><span id="MJXc-Node-19" class="mjx-mn"><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 id="MJXc-Node-20" class="mjx-mo"><span class="mjx-char"></span></span><span id="MJXc-Node-21" class="mjx-mn MJXc-space1"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.396em; padding-bottom: 0.347em;">5</span></span><span id="MJXc-Node-22" 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-23" class="mjx-mi MJXc-space3"><span class="mjx-char MJXc-TeX-math-I" style="padding-top: 0.495em; padding-bottom: 0.298em;">b</span></span></span></span></span></span><span class="MJX_Assistive_MathML" role="presentation"><math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle mathsize="1.44em"><msub><mi>log</mi><mrow class="MJX-TeXAtom-ORD"><mn>14</mn></mrow></msub><mo>⁡</mo><mn>7</mn><mo>=</mo><mi>a</mi><mo>;</mo><msub><mi>log</mi><mrow class="MJX-TeXAtom-ORD"><mn>14</mn></mrow></msub><mo>⁡</mo><mn>5</mn><mo>=</mo><mi>b</mi></mstyle></math></span></span><script type="math/tex" id="MathJax-Element-1">\Large \log_{14}7=a; \log_{14}5=b</script>. Tính <span class="MathJax_Preview" style="color: inherit; display: none;"></span><span id="MathJax-Element-2-Frame" class="mjx-chtml MathJax_CHTML" tabindex="0" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle mathsize="1.44em"><msub><mi>log</mi><mrow class="MJX-TeXAtom-ORD"><mn>35</mn></mrow></msub><mo>&#x2061;</mo><mn>28</mn></mstyle></math>" role="presentation" style="font-size: 127%; position: relative;"><span id="MJXc-Node-24" class="mjx-math" aria-hidden="true"><span id="MJXc-Node-25" class="mjx-mrow"><span id="MJXc-Node-26" class="mjx-mstyle"><span id="MJXc-Node-27" class="mjx-mrow" style="font-size: 144%;"><span id="MJXc-Node-28" class="mjx-msubsup"><span class="mjx-base"><span id="MJXc-Node-29" class="mjx-mi"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.396em; padding-bottom: 0.544em;">log</span></span></span><span class="mjx-sub" style="font-size: 70.7%; vertical-align: -0.377em; padding-right: 0.071em;"><span id="MJXc-Node-30" class="mjx-texatom" style=""><span id="MJXc-Node-31" class="mjx-mrow"><span id="MJXc-Node-32" class="mjx-mn"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.396em; padding-bottom: 0.347em;">35</span></span></span></span></span></span><span id="MJXc-Node-33" class="mjx-mo"><span class="mjx-char"></span></span><span id="MJXc-Node-34" class="mjx-mn MJXc-space1"><span class="mjx-char MJXc-TeX-main-R" style="padding-top: 0.396em; padding-bottom: 0.347em;">28</span></span></span></span></span></span><span class="MJX_Assistive_MathML" role="presentation"><math xmlns="http://www.w3.org/1998/Math/MathML"><mstyle mathsize="1.44em"><msub><mi>log</mi><mrow class="MJX-TeXAtom-ORD"><mn>35</mn></mrow></msub><mo>⁡</mo><mn>28</mn></mstyle></math></span></span><script type="math/tex" id="MathJax-Element-2">\Large \log_{35}28</script> the

Cho log147=a;log145=b. Tính log3528 the

4.7/5

Tác giả: Thầy Tùng

Đăng ngày: 18 Aug 2022

Lưu về Facebook:

Câu hỏi:

Cho log147=a;log145=b. Tính log3528 theo a, b

Đáp án án đúng là: A

Lời giải chi tiết:

Chọn A

Vì logarit cần tính không cùng cơ số với hai logarit đã cho nên ta sẽ đổi logarit cần tính về cơ số 14

Ta có: log3528=log1428log1435

Lại thấy: log1435=log14(7.5)=log147+log145=a+b

Tiếp theo ta biểu diễn log1428 theo các logarit log157log145. Muốn vậy ta chỉ biểu diễn số 28 thành tích hoặc thương của các lũy thừa của 14, 7 và 5 là xong 

Ta có: 28=14.2=14.147=1427

Suy ra log1428=log141427=log14142log147=2a

Vậy log3528=2aa+b