__4__ + __4__ + __4__ + .........___4__ 3*5 5*7 7*9 99 *101
2 câu trả lời
.\(\begin{array}{l}\frac{4}{{3 \times 5}} + \frac{4}{{5 \times 7}} + \frac{4}{{7 \times 9}} + ... + \frac{4}{{99 \times 101}}\\ = \frac{1}{2} \times 4 \times \left( {\frac{1}{3} - \frac{1}{5} + \frac{1}{5} - \frac{1}{7} + \frac{1}{7} - \frac{1}{9} + .... + \frac{1}{{99}} - \frac{1}{{101}}} \right)\\ = 2 \times \left( {\frac{1}{3} - \frac{1}{{101}}} \right)\\ = 2 \times \frac{{98}}{{303}}\\ = \frac{{196}}{{303}}\end{array}\).
Đáp án:
A = $\frac{196}{303}$
Giải thích các bước giải:
A = $\frac{1}{2}$ (($\frac{4}{3}$ - $\frac{4}{5}$ ) + ($\frac{4}{5}$ - $\frac{4}{7}$ ) + (($\frac{4}{7}$ - $\frac{4}{9}$ )) + ..... + ($\frac{4}{99}$ - $\frac{4}{101}$ )
A = $\frac{1}{2}$ (($\frac{4}{3}$ - $\frac{4}{101}$ ))
A= $\frac{1}{2}$ $\frac{392}{303}$
A = $\frac{196}{303}$