№2 Вычислить: (1×1-2+1)÷(2-1)

$$\left(1\frac{17}{18} \times 1\frac{13}{14}-2\frac{5}{8} \div 1\frac{19}{20}\right) \div \left(2\frac{25}{78} - 1\frac{1}{26}\right)$$

1) $$1\frac{17}{18} \times 1\frac{13}{14} = \frac{35}{18} \times \frac{27}{14} = \frac{5}{2} \times \frac{3}{2} = \frac{15}{4} = 3\frac{3}{4}$$

2) $$2\frac{5}{8} \div 1\frac{19}{20} = \frac{21}{8} \div \frac{39}{20} = \frac{21}{8} \times \frac{20}{39} = \frac{7}{2} \times \frac{5}{13} = \frac{35}{26} = 1\frac{9}{26}$$

3) $$3\frac{3}{4} - 1\frac{9}{26} = 3\frac{39}{52} - 1\frac{18}{52} = 2\frac{21}{52} = \frac{125}{52}$$

4) $$2\frac{25}{78} - 1\frac{1}{26} = 2\frac{25}{78} - 1\frac{3}{78} = 1\frac{22}{78} = 1\frac{11}{39} = \frac{50}{39}$$

5) $$\frac{125}{52} \div \frac{50}{39} = \frac{125}{52} \times \frac{39}{50} = \frac{5}{4} \times \frac{3}{2} = \frac{15}{8} = 1\frac{7}{8}$$

Ответ: $$1\frac{7}{8}$$