Tính\[{\rm{A}} = \left( {1 - \frac{1}{{{2^2}}}} \right)\left( {1 - \frac{1}{{{3^2}}}} \right) \cdot \cdot \cdot \left( {1 - \frac{1}{{{{2010}^2}}}} \right)\]

Lời giải

Đáp án đúng là: C

\[\left( {1 - \frac{1}{{{2^2}}}} \right)\left( {1 - \frac{1}{{{3^2}}}} \right)\left( {1 - \frac{1}{{{4^2}}}} \right)\left( {1 - \frac{1}{{{5^2}}}} \right) \cdot \cdot \cdot \left( {1 - \frac{1}{{{n^2}}}} \right)\]

\[ = \frac{{{2^2} - 1}}{{{2^2}}} \cdot \frac{{{3^2} - 1}}{{{3^2}}} \cdot \frac{{{4^2} - 1}}{{{4^2}}} \cdot \frac{{{5^2} - 1}}{{{5^2}}} \cdots \frac{{{n^2} - 1}}{{{n^2}}}\]

\[ = \frac{{1.\,3}}{{{2^2}}} \cdot \frac{{2\,.\,4}}{{{3^2}}} \cdot \frac{{3\,.\,5}}{{{4^2}}} \cdot \frac{{4\,.\,6}}{{{5^2}}} \cdots \frac{{\left( {n - 1} \right)\left( {n + 1} \right)}}{{{n^2}}}\]

\( = \frac{{1.2.3.4...\left( {n - 1} \right)}}{{2.3.4.5...n}}.\frac{{3.4.5.6...\left( {n + 1} \right)}}{{2.3.4.5...n}}\)

\( = \frac{1}{n}.\frac{{n + 1}}{2} = \frac{{n + 1}}{{2n}}\)

Áp dụng với n = 2010, ta có:

\[{\rm{A}} = \left( {1 - \frac{1}{{{2^2}}}} \right)\left( {1 - \frac{1}{{{3^2}}}} \right) \cdot \cdot \cdot \left( {1 - \frac{1}{{{{2010}^2}}}} \right) = \frac{{2010 + 1}}{{2\,.\,2010}} = \frac{{2011}}{{4020}}\].