Tính: 9/1*4 9/4*7 ...
Giải thích
\[B = \frac{9}{{1 \cdot 4}} + \frac{9}{{4 \cdot 7}} + ... + \frac{9}{{97 \cdot 100}}\]
\[ = 3 \cdot \left( {\frac{3}{{1 \cdot 4}} + \frac{3}{{4 \cdot 7}} + ... + \frac{3}{{97 \cdot 100}}} \right)\]
\[ = 3\left( {1 - \frac{1}{4} + \frac{1}{4} - \frac{1}{7} + ... + \frac{1}{{97}} - \frac{1}{{100}}} \right)\]
\[ = 3\left( {1 - \frac{1}{{100}}} \right)\]
\[ = 3 \cdot \frac{{99}}{{100}} = \frac{{297}}{{100}}.\]