Tính: a) 1/3 + 4/7 + 5/21 - 1 = ......... b) (2 2/7 + 1/6) * 1 2/5 - 3

* Hướng dẫn giải

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

b) $\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$$=\left(\frac{96}{42}+\frac{7}{42}\right)\times \frac{7}{5}-3=\frac{103}{42}\times \frac{7}{5}-3$$=\frac{103}{30}-\frac{90}{30}=\frac{13}{30}$