Выполним сложение дробей.
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\(\frac{1}{1} + \frac{1}{12} = \frac{12}{12} + \frac{1}{12} = \frac{12+1}{12} = \frac{13}{12}\)
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\(\frac{1}{6} + \frac{1}{3} = \frac{1}{6} + \frac{1 \cdot 2}{3 \cdot 2} = \frac{1}{6} + \frac{2}{6} = \frac{1+2}{6} = \frac{3}{6} = \frac{1}{2}\)
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\(\frac{7}{8} + \frac{3}{4} = \frac{7}{8} + \frac{3 \cdot 2}{4 \cdot 2} = \frac{7}{8} + \frac{6}{8} = \frac{7+6}{8} = \frac{13}{8}\)
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\(\frac{4}{7} + \frac{2}{21} = \frac{4 \cdot 3}{7 \cdot 3} + \frac{2}{21} = \frac{12}{21} + \frac{2}{21} = \frac{12+2}{21} = \frac{14}{21} = \frac{2}{3}\)
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\(\frac{9}{12} + \frac{3}{4} = \frac{9}{12} + \frac{3 \cdot 3}{4 \cdot 3} = \frac{9}{12} + \frac{9}{12} = \frac{9+9}{12} = \frac{18}{12} = \frac{3}{2}\)
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\(\frac{8}{9} + \frac{1}{3} = \frac{8}{9} + \frac{1 \cdot 3}{3 \cdot 3} = \frac{8}{9} + \frac{3}{9} = \frac{8+3}{9} = \frac{11}{9}\)
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\(\frac{4}{4} + \frac{7}{15} = \frac{4 \cdot 15}{4 \cdot 15} + \frac{7}{15} = \frac{60}{15} + \frac{7}{15} = \frac{60+7}{15} = \frac{67}{15}\)
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\(\frac{2}{21} + \frac{5}{14} = \frac{2 \cdot 2}{21 \cdot 2} + \frac{5 \cdot 3}{14 \cdot 3} = \frac{4}{42} + \frac{15}{42} = \frac{4+15}{42} = \frac{19}{42}\)
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\(\frac{3}{18} + \frac{5}{27} = \frac{3 \cdot 3}{18 \cdot 3} + \frac{5 \cdot 2}{27 \cdot 2} = \frac{9}{54} + \frac{10}{54} = \frac{9+10}{54} = \frac{19}{54}\)
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\(\frac{3}{15} + \frac{7}{20} = \frac{3 \cdot 4}{15 \cdot 4} + \frac{7 \cdot 3}{20 \cdot 3} = \frac{12}{60} + \frac{21}{60} = \frac{12+21}{60} = \frac{33}{60} = \frac{11}{20}\)
Ответ: смотри решение выше