Tính \(S = \left( {1 - \frac{1}{{{2^2}}}} \right)\left( {1 - \frac{1}{{{3^2}}}} \right)\left( {1 - \frac{1}{{{4^2}}}} \right)\left( {1 - \frac{1}{{{5^2}}}} \right)\left( {1 - \frac{1}{{{6^2}}}} \right)...\left( {1 - \frac{1}{{{{99}^2}}}} \right)\).
Rút gọn A, biến đổi các phân số trong A để rút gọn.
\(\begin{array}{l}S = \left( {1 - \frac{1}{4}} \right).\left( {1 - \frac{1}{9}} \right).\left( {1 - \frac{1}{{16}}} \right).\left( {1 - \frac{1}{{25}}} \right)\left( {1 - \frac{1}{{36}}} \right)...\left( {1 - \frac{1}{{9901}}} \right)\\ = \frac{3}{4} \cdot \frac{8}{9} \cdot \frac{{15}}{{16}} \cdot \frac{{24}}{{25}} \cdot \frac{{35}}{{36}} \cdots \frac{{9800}}{{99}}\\ = \frac{{1.3}}{{2.2}} \cdot \frac{{2.4}}{{3.3}} \cdot \frac{{3.5}}{{4.4}} \cdot \frac{{4.6}}{{5.5}} \cdot \frac{{5.7}}{{6.6}} \cdots \frac{{98.100}}{{99.99}}\\ = \frac{{1.2.3.4.5...98}}{{2.3.4.5.6...99}} \cdot \frac{{3.4.5.6.7...100}}{{2.3.4.5.6...99}}\\ = \frac{1}{{99}} \cdot \frac{{100}}{2}\\ = \frac{{50}}{{99}} \cdot \end{array}\)
Vậy \(S = \frac{{50}}{{99}}\).