Rút gọn biểu thức sau: A = ( 1 - 1/2^2)( 1 - 1/3^2)...( 1 - 1/n^2) A. n + 1/2n B. n - 1/2n C. n/n - 1 D. n/n + 1
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14/09/2024
Rút gọn biểu thức sau: \[{\rm{A}} = \left( {1 - \frac{1}{{{2^2}}}} \right)\left( {1 - \frac{1}{{{3^2}}}} \right)...\left( {1 - \frac{1}{{{n^2}}}} \right)\]
A. \(\frac{{n + 1}}{{2n}}\)
B. \(\frac{{n - 1}}{{2n}}\)
C. \(\frac{n}{{n - 1}}\)
D. \(\frac{n}{{n + 1}}\)
Trả lời
Lời giải
Đáp án đúng là: A
\[{\rm{A}} = \left( {1 - \frac{1}{{{2^2}}}} \right)\left( {1 - \frac{1}{{{3^2}}}} \right)...\left( {1 - \frac{1}{{{n^2}}}} \right)\]
\[ = \frac{{{2^2} - 1}}{{{2^2}}} \cdot \frac{{{3^2} - 1}}{{{3^2}}} \cdot \frac{{{4^2} - 1}}{{{4^2}}} \cdot \frac{{{5^2} - 1}}{{{5^2}}} \cdots \frac{{{n^2} - 1}}{{{n^2}}}\]
\( = \frac{{1.3}}{{{2^2}}}.\frac{{2.4}}{{{3^2}}}.\frac{{3.5}}{{{4^2}}}.\frac{{4.6}}{{{5^2}}}...\frac{{\left( {n - 1} \right)\left( {n + 1} \right)}}{{{n^2}}}\)
\( = \frac{{1.2.3.4...\left( {n - 1} \right)}}{{2.3.4.5...n}}.\frac{{3.4.5.6...\left( {n + 1} \right)}}{{2.3.4.5...n}}\)
\( = \frac{1}{n}.\frac{{n + 1}}{2} = \frac{{n + 1}}{{2n}}\)
