442. a) $$\frac{5}{9} + \frac{2}{3}$$; б) $$\frac{1}{12} + \frac{3}{4}$$; в) $$\frac{1}{2} + \frac{3}{8}$$; г) $$\frac{17}{1000} + \frac{81}{100}$$

a) $$\frac{5}{9} + \frac{2}{3} = \frac{5}{9} + \frac{2 \cdot 3}{3 \cdot 3} = \frac{5}{9} + \frac{6}{9} = \frac{5+6}{9} = \frac{11}{9} = 1\frac{2}{9}$$ б) $$\frac{1}{12} + \frac{3}{4} = \frac{1}{12} + \frac{3 \cdot 3}{4 \cdot 3} = \frac{1}{12} + \frac{9}{12} = \frac{1+9}{12} = \frac{10}{12} = \frac{5}{6}$$ в) $$\frac{1}{2} + \frac{3}{8} = \frac{1 \cdot 4}{2 \cdot 4} + \frac{3}{8} = \frac{4}{8} + \frac{3}{8} = \frac{4+3}{8} = \frac{7}{8}$$ г) $$\frac{17}{1000} + \frac{81}{100} = \frac{17}{1000} + \frac{81 \cdot 10}{100 \cdot 10} = \frac{17}{1000} + \frac{810}{1000} = \frac{17+810}{1000} = \frac{827}{1000}$$ Ответ: a) $$1\frac{2}{9}$$; б) $$\frac{5}{6}$$; в) $$\frac{7}{8}$$; г) $$\frac{827}{1000}$$