4 4 5 12—+3—= 11 11 5 6 8—+7—= 17 17 9 6—+3—= 19 19

Выполним сложение дробей:

1. $$12 \frac{4}{11} + 3 \frac{5}{11} = (12+3) + (\frac{4}{11} + \frac{5}{11}) = 15 + \frac{4+5}{11} = 15 + \frac{9}{11} = 15\frac{9}{11}$$
2. $$8 \frac{9}{17} + 7 \frac{6}{17} = (8+7) + (\frac{9}{17} + \frac{6}{17}) = 15 + \frac{9+6}{17} = 15 + \frac{15}{17} = 15\frac{15}{17}$$
3. $$6 \frac{9}{19} + 3 \frac{9}{19} = (6+3) + (\frac{9}{19} + \frac{9}{19}) = 9 + \frac{9+9}{19} = 9 + \frac{18}{19} = 9\frac{18}{19}$$

Ответ: 1) $$15\frac{9}{11}$$, 2) $$15\frac{15}{17}$$, 3) $$9\frac{18}{19}$$