Thực hiện các phép tính (tính hợp lý nếu có thể) a) 2/3(3/4-5/7)-1/28:(-5/6)+1/3) b) 12/37+3/21+25/37-7/14+6/7

a) $\frac{2}{3}\cdot (\frac{3}{4}-\frac{5}{7})-\frac{1}{28}:(\frac{-5}{6}+\frac{1}{3})$

$=\frac{2}{3}\cdot (\frac{21}{28}-\frac{20}{28})-\frac{1}{28}:(\frac{-5}{6}+\frac{2}{6})$

$=\frac{2}{3}\cdot \frac{1}{28}-\frac{1}{28}:\left(\frac{-3}{6}\right)$

$=\frac{2}{3}\cdot \frac{1}{28}+\frac{1}{28}\cdot 2$

$=\frac{1}{28}\cdot (\frac{2}{3}+2)$

b) $\frac{12}{37}+\frac{3}{21}+\frac{25}{37}-\frac{7}{14}+\frac{6}{7}$

$=(\frac{12}{37}+\frac{25}{37})+(\frac{3}{21}+\frac{6}{7})-\frac{7}{14}$ 

$=\frac{37}{37}+(\frac{3}{21}+\frac{18}{21})-\frac{1}{2}$ 

$=1+1-\frac{1}{2}$

$=2-\frac{1}{2}=\frac{3}{2}$.