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512)
х = 0,5
у = 1,5

513)
$1)( \frac{5}{21} + \frac{2}{15} ) + \frac{2}{21} = \frac{25}{105} + \frac{14}{105} + \frac{10}{105} = \frac{49}{105} = \frac{7}{15} \\ \\ 2)( \frac{2}{35} + \frac{3}{28} ) + \frac{1}{28} = \frac{8}{140} + \frac{15}{140} + \frac{5}{140} = \frac{28}{140} = \frac{1}{5} \\ \\ 3)( \frac{7}{30} + \frac{8}{45} ) + ( \frac{2}{45} + \frac{1}{30} ) = \frac{21}{90} + \frac{16}{90} + \frac{4}{90} + \frac{3}{90} = \frac{44}{90} = \frac{22}{45} \\ \\ 4)( \frac{5}{24} + \frac{11}{60} ) + ( \frac{13}{60} + \frac{1}{24} ) = \frac{25}{120} + \frac{22}{120} + \frac{26}{120} + \frac{5}{120} = \frac{78}{120} = \frac{13}{20}$