a)  \(\frac{2}{3}\) + \(\frac{3}{5}\)  = \(\frac{2 \times 5}{3 \times 5}\) + \(\frac{3 \times 3}{5 \times 3}\) 

= \(\frac{10}{15}\) + \(\frac{9}{15}\) = \(\frac{19}{15}\)

\(\frac{7}{12}\) - \(\frac{2}{7}\) + \(\frac{1}{12}\) = (\(\frac{7}{12}\) + \(\frac{1}{12}\)) - \(\frac{2}{7}\) = \(\frac{8}{12}\) - \(\frac{2}{7}\) = \(\frac{2}{3}\) - \(\frac{2}{7}\) = \(\frac{2 \times 7}{3 \times 7}\) - \(\frac{2 \times 3}{7 \times 3}\) = \(\frac{14}{21}\) - \(\frac{6}{21}\)

= \(\frac{8}{21}\).

\(\frac{12}{17}\) - \(\frac{5}{17}\) - \(\frac{4}{17}\)  = \(\frac{12}{17}\) - (\(\frac{5}{17}\)  + \(\frac{4}{17}\)) =  \(\frac{12}{17}\) - \(\frac{9}{17}\) = \(\frac{3}{17}\) ;

b) 

 578,69 + 281,78 = 860,47.

594,72 + 406,38 - 329,47 = 1001,1 - 329,47 = 671,63.