Nối phép tính đúng: 3/7 + 5/21

Nối.

Giải thích

\[\frac{3}{7} + \frac{5}{{21}} = \frac{9}{{21}} + \frac{5}{{21}} = \frac{{14}}{{21}}\]

\[\frac{4}{5} + \frac{{13}}{{35}} = \frac{{28}}{{35}} + \frac{{13}}{{35}} = \frac{{41}}{{35}}\]

\[\frac{7}{6} + \frac{5}{{42}} = \frac{{49}}{{42}} + \frac{5}{{42}} = \frac{{54}}{{42}} = \frac{9}{7}\]

\[\frac{9}{{40}} + \frac{3}{8} = \frac{9}{{40}} + \frac{{15}}{{40}} = \frac{{24}}{{40}} = \frac{3}{5}\]