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