Đáp án:
Giải thích các bước giải:
$M=\dfrac{3}{2.5}+\dfrac{3}{5.8}+\dfrac{3}{8.11}+\dfrac{3}{11.14}+\dfrac{3}{14.13}$
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$M=\dfrac{1}{2}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{14}+\dfrac{1}{14}-\dfrac{1}{13}$
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$M=\dfrac{1}{2}-\dfrac{1}{13}$
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$M=\dfrac{13}{26}-\dfrac{2}{26}$
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$M=\dfrac{11}{26}$