2.199 Вычислите: а) (2/5 − 1/4) + 9/20; б) 7/30 + (3/5 − 1/6); в) 7/8 − (1/9 + 2/3); г) (5/14 + 9/10) − 5/7.

а)

\(\left(\frac{2}{5}-\frac{1}{4}\right)+\frac{9}{20}=\left(\frac{8}{20}-\frac{5}{20}\right)+\frac{9}{20}=\frac{3}{20}+\frac{9}{20}=\frac{12}{20}=\frac{3}{5}\)

б)

\(\frac{7}{30}+\left(\frac{3}{5}-\frac{1}{6}\right)=\frac{7}{30}+\left(\frac{18}{30}-\frac{5}{30}\right)=\frac{7}{30}+\frac{13}{30}=\frac{20}{30}=\frac{2}{3}\)

в)

\(\frac{7}{8}-\left(\frac{1}{9}+\frac{2}{3}\right)=\frac{7}{8}-\left(\frac{1}{9}+\frac{6}{9}\right)=\frac{7}{8}-\frac{7}{9}=\frac{63}{72}-\frac{56}{72}=\frac{7}{72}\)

г)

\(\left(\frac{5}{14}+\frac{9}{10}\right)-\frac{5}{7}=\frac{25}{70}+\frac{63}{70}-\frac{50}{70}=\frac{38}{70}=\frac{19}{35}\)

Ответ: а) \(\frac{3}{5}\); б) \(\frac{2}{3}\); в) \(\frac{7}{72}\); г) \(\frac{19}{35}\).