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Ответ:
1) $$9 \frac{3}{4} + 3 \frac{1}{6} = 9 + \frac{3}{4} + 3 + \frac{1}{6} = (9 + 3) + (\frac{3}{4} + \frac{1}{6}) = 12 + (\frac{3 \cdot 3}{4 \cdot 3} + \frac{1 \cdot 2}{6 \cdot 2}) = 12 + (\frac{9}{12} + \frac{2}{12}) = 12 + \frac{11}{12} = 12 \frac{11}{12}$$
2) $$11 \frac{7}{12} + 6 \frac{11}{18} = 11 + \frac{7}{12} + 6 + \frac{11}{18} = (11 + 6) + (\frac{7}{12} + \frac{11}{18}) = 17 + (\frac{7 \cdot 3}{12 \cdot 3} + \frac{11 \cdot 2}{18 \cdot 2}) = 17 + (\frac{21}{36} + \frac{22}{36}) = 17 + \frac{43}{36} = 17 + 1 \frac{7}{36} = 18 \frac{7}{36}$$
3) $$7 \frac{8}{15} + 8 \frac{17}{20} = 7 + \frac{8}{15} + 8 + \frac{17}{20} = (7 + 8) + (\frac{8}{15} + \frac{17}{20}) = 15 + (\frac{8 \cdot 4}{15 \cdot 4} + \frac{17 \cdot 3}{20 \cdot 3}) = 15 + (\frac{32}{60} + \frac{51}{60}) = 15 + \frac{83}{60} = 15 + 1 \frac{23}{60} = 16 \frac{23}{60}$$
4) $$7 \frac{3}{8} + 9 \frac{7}{16} + 11 \frac{7}{12} = 7 + \frac{3}{8} + 9 + \frac{7}{16} + 11 + \frac{7}{12} = (7 + 9 + 11) + (\frac{3}{8} + \frac{7}{16} + \frac{7}{12}) = 27 + (\frac{3 \cdot 6}{8 \cdot 6} + \frac{7 \cdot 3}{16 \cdot 3} + \frac{7 \cdot 4}{12 \cdot 4}) = 27 + (\frac{18}{48} + \frac{21}{48} + \frac{28}{48}) = 27 + \frac{67}{48} = 27 + 1 \frac{19}{48} = 28 \frac{19}{48}$$
Ответ: 1) 12 11/12; 2) 18 7/36; 3) 16 23/60; 4) 28 19/48
