Tính: c) [ 3/5 + ( − 1/4 ) ] : 3/7 + [ ( − 3/4 ) + 2/5 ] : 3/7 ;

c) $[\frac{3}{5}+\left(\frac{-1}{4}\right)]:\frac{3}{7}+[\left(\frac{-3}{4}\right)+\frac{2}{5}]:\frac{3}{7}$

$=[\frac{3}{5}+\left(\frac{-1}{4}\right)].\frac{7}{3}+[\left(\frac{-3}{4}\right)+\frac{2}{5}].\frac{7}{3}$

$=[\frac{3}{5}+\left(\frac{-1}{4}\right)+\left(\frac{-3}{4}\right)+\frac{2}{5}].\frac{7}{3}$

$=[(\frac{3}{5}+\frac{2}{5})+(\frac{-1}{4}+\frac{-3}{4})].\frac{7}{3}$

$=[1+(-1)].\frac{7}{3}=0.\frac{7}{3}=0$