Tìm x, biết: a) 3/7 + x = 4/5 ;

$\begin{array}{l}\mathrm{a}) \frac{3}{7}+\mathrm{x}=\frac{4}{5}\\ \mathrm{x}=\frac{4}{5}-\frac{3}{7}\\ \mathrm{x}=\frac{4.7}{5.7}-\frac{3.5}{7.5}\\ \mathrm{x}=\frac{28}{35}-\frac{15}{35}\\ \mathrm{x}=\frac{13}{35}\\ \mathrm{Vậy} \mathrm{x}=\frac{13}{35}\end{array}$