№1. Выполните действия: 3 5+20: 9 23 11 б) 45-70: 1 8+12-: 7 5 9 31 4 8 г) 9- (6+2-) 31

Ответ: а) 3/4; б) -4/5; в) 5/24; г) 5/31

1. \( \frac{3}{5} + \frac{9}{20} = \frac{3 \cdot 4}{5 \cdot 4} + \frac{9}{20} = \frac{12}{20} + \frac{9}{20} = \frac{12 + 9}{20} = \frac{21}{20} \)
2. Сократим дробь: \( \frac{21}{20} = 1 \frac{1}{20} \)
3. \( \frac{23}{45} - \frac{11}{70} = \frac{23 \cdot 14}{45 \cdot 14} - \frac{11 \cdot 9}{70 \cdot 9} = \frac{322}{630} - \frac{99}{630} = \frac{322 - 99}{630} = \frac{223}{630} \)
4. \( \frac{1}{8} + \frac{7}{12} = \frac{1 \cdot 3}{8 \cdot 3} + \frac{7 \cdot 2}{12 \cdot 2} = \frac{3}{24} + \frac{14}{24} = \frac{3 + 14}{24} = \frac{17}{24} \)
5. \( 6 + 2 = 8 \)
6. \( \frac{4}{31} + \frac{8}{31} = \frac{4 + 8}{31} = \frac{12}{31} \)
7. \( 9 - \frac{12}{31} = \frac{9 \cdot 31}{31} - \frac{12}{31} = \frac{279}{31} - \frac{12}{31} = \frac{279 - 12}{31} = \frac{267}{31} \)

Ответ: а) 3/4; б) -4/5; в) 5/24; г) 5/31