2 câu trả lời
`2/(1xx3) ++ 2/(3xx5) + 2/(5xx7) + 2/(7xx9) + 2/(9xx11)`
`= 1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + 1/7- 1/9 + 1/9 - 1/11`
`= 1 - 1/11`
`= 11/11 - 1/11`
`= 10/11`
Vậy `2/(1xx3) + 2/(3xx5) + 2/(5xx7) + 2/(7xx9) + 2/(9xx11) = 10/11`
Đáp án:
$\dfrac{10}{11}$
Lời giải:
\(\begin{array}{l}
\dfrac{2}{{1 \times 3}} + \dfrac{2}{{3 \times 5}} + \dfrac{2}{{5 \times 7}} + \dfrac{2}{{7 \times 9}} + \dfrac{2}{{9 \times 11}}\\
= 1 - \dfrac{1}{3} + \dfrac{1}{3} - \dfrac{1}{5} + \dfrac{1}{5} - \dfrac{1}{7} + \dfrac{1}{7} - \dfrac{1}{9} + \dfrac{1}{9} - \dfrac{1}{{11}}\\
= 1 - \dfrac{1}{{11}}\\
= \dfrac{{10}}{{11}}
\end{array}\)
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