Ai giỏi toán thì giải nhé

S

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$\dfrac{1}{2.6}+\dfrac{1}{4.9}+...+\dfrac{1}{38.60}$

$=\dfrac{1}{6}(\dfrac{1}{1.2}+\dfrac{1}{2.3}+...+ \dfrac{ 1 } { 19.20 } )$

$=\dfrac{1}{6}(1-\dfrac{1}{20})=\dfrac{19}{120}$
 
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