а) 1/7 + 2/21 = ; б) 8/9 − 2/9 = ; в) 1/7 + 2/16 = ; г) 8/9 − 1/3 =

1. \(\frac{1}{7}+\frac{2}{21}=\frac{3}{21}+\frac{2}{21}=\frac{5}{21}\).
2. \(\frac{8}{9}-\frac{2}{9}=\frac{6}{9}=\frac{2}{3}\).
3. \(\frac{1}{7}+\frac{2}{16}=\frac{1}{7}+\frac{1}{8}=\frac{8}{56}+\frac{7}{56}=\frac{15}{56}\).
4. \(\frac{8}{9}-\frac{1}{3}=\frac{8}{9}-\frac{3}{9}=\frac{5}{9}\).

Ответ: а) \(\frac{5}{21}\); б) \(\frac{2}{3}\); в) \(\frac{15}{56}\); г) \(\frac{5}{9}\).