tính: (1/3) + (4/7) + (5/21)-1 =

* Hướng dẫn giải

$\frac{1}{3}+\frac{4}{7}+\frac{5}{21}-1=\frac{7}{21}+\frac{12}{21}+\frac{5}{21}-\frac{21}{21}=\frac{3}{21}=\frac{1}{7}$

$$\begin{gathered} \left(2\frac{2}{7}+\frac{1}{6}\right)\times 1\frac{2}{5}-3=\left(\frac{16}{7}+\frac{1}{6}\right)\times \frac{7}{5}-3 \\ =\frac{103}{42}\times \frac{7}{5}-3=\frac{103}{30}-\frac{90}{30}=\frac{13}{30} \end{gathered}$$