в) $$\frac{17}{42}c - \frac{2}{7}c + \frac{7}{18}c$$ при $$c = 3 \frac{1}{2}; 2 \frac{5}{8};$$

в) $$\frac{17}{42}c - \frac{2}{7}c + \frac{7}{18}c$$ при $$c = 3 \frac{1}{2} = \frac{7}{2}$$:

$$\frac{17}{42} \cdot \frac{7}{2} - \frac{2}{7} \cdot \frac{7}{2} + \frac{7}{18} \cdot \frac{7}{2} = \frac{17}{6} \cdot \frac{1}{2} - \frac{1}{1} \cdot \frac{1}{1} + \frac{7}{18} \cdot \frac{7}{2} = \frac{17}{12} - 1 + \frac{49}{36} = \frac{17 \cdot 3}{12 \cdot 3} - \frac{1 \cdot 36}{1 \cdot 36} + \frac{49}{36} = \frac{51}{36} - \frac{36}{36} + \frac{49}{36} = \frac{51 - 36 + 49}{36} = \frac{64}{36} = \frac{16}{9} = 1 \frac{7}{9}$$

в) $$\frac{17}{42}c - \frac{2}{7}c + \frac{7}{18}c$$ при $$c = 2 \frac{5}{8} = \frac{21}{8}$$:

$$\frac{17}{42} \cdot \frac{21}{8} - \frac{2}{7} \cdot \frac{21}{8} + \frac{7}{18} \cdot \frac{21}{8} = \frac{17}{2} \cdot \frac{1}{8} - \frac{1}{1} \cdot \frac{3}{8} + \frac{7}{2} \cdot \frac{7}{8} = \frac{17}{16} - \frac{3}{4} + \frac{49}{16} = \frac{17}{16} - \frac{3 \cdot 4}{4 \cdot 4} + \frac{49}{16} = \frac{17}{16} - \frac{12}{16} + \frac{49}{16} = \frac{17 - 12 + 49}{16} = \frac{54}{16} = \frac{27}{8} = 3 \frac{3}{8}$$

Ответ: Если $$c = 3 \frac{1}{2}$$, то $$\frac{17}{42}c - \frac{2}{7}c + \frac{7}{18}c = 1 \frac{7}{9}$$. Если $$c = 2 \frac{5}{8}$$, то $$\frac{17}{42}c - \frac{2}{7}c + \frac{7}{18}c = 3 \frac{3}{8}$$