A=34/7×13 + 51/13×22 +85/22×37 +68/37×49 help me plzzzzz

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1. Đáp án:

   $A=\dfrac{34}{49}$ 

   Giải thích các bước giải:

    $A=\dfrac{34}{7.13}+\dfrac{51}{13.22}+\dfrac{85}{22.37}+\dfrac{68}{37.49}\\
   =\dfrac{17}{3}\left ( \dfrac{6}{7.13}+\dfrac{9}{13.22} +\dfrac{15}{22.37}+\dfrac{12}{37.49}\right )\\
   =\dfrac{17}{3}\left ( \dfrac{1}{7}-\dfrac{1}{13}+\dfrac{1}{13}-\dfrac{1}{22}+\dfrac{1}{22}-\dfrac{1}{37}+\dfrac{1}{37}-\dfrac{1}{49} \right )\\
   =\dfrac{17}{3}\left ( \dfrac{1}{7}-\dfrac{1}{49} \right )\\
   =\dfrac{17}{3}.\left ( \dfrac{7}{49}-\dfrac{1}{49} \right )\\
   =\dfrac{17}{3}.\dfrac{6}{49}\\
   =\dfrac{34}{49}$

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