172. Найдите сумму: 1)-7+(-8); 2)-0,38+(-0,74); 3)-1,8+(-0,34); 4)-4$$\frac{6}{7}$$+(-2$$\frac{3}{7}$$); 5)-$$\frac{5}{6}$$+(-$$\frac{7}{8}$$); 6)-6$$\frac{7}{15}$$+(-8$$\frac{3}{10}$$).

1) -7 + (-8) = -7 - 8 = -15

2) -0,38 + (-0,74) = -0,38 - 0,74 = -1,12

3) -1,8 + (-0,34) = -1,8 - 0,34 = -2,14

4) $$-4\frac{6}{7} + (-2\frac{3}{7}) = -4\frac{6}{7} - 2\frac{3}{7} = - (4\frac{6}{7} + 2\frac{3}{7}) = - (4 + 2 + \frac{6}{7} + \frac{3}{7}) = - (6 + \frac{6+3}{7}) = - (6 + \frac{9}{7}) = - (6 + 1\frac{2}{7}) = -7\frac{2}{7}$$

5) $$- \frac{5}{6} + (- \frac{7}{8}) = - \frac{5}{6} - \frac{7}{8} = - (\frac{5}{6} + \frac{7}{8})$$. Общий знаменатель 24. $$- (\frac{5 \cdot 4}{6 \cdot 4} + \frac{7 \cdot 3}{8 \cdot 3}) = - (\frac{20}{24} + \frac{21}{24}) = - \frac{20+21}{24} = - \frac{41}{24} = -1\frac{17}{24}$$

6) $$-6\frac{7}{15} + (-8\frac{3}{10}) = -6\frac{7}{15} - 8\frac{3}{10} = -(6\frac{7}{15} + 8\frac{3}{10}) = - (6 + 8 + \frac{7}{15} + \frac{3}{10}) = - (14 + \frac{7}{15} + \frac{3}{10})$$. Общий знаменатель 30. $$-(14 + \frac{7 \cdot 2}{15 \cdot 2} + \frac{3 \cdot 3}{10 \cdot 3}) = - (14 + \frac{14}{30} + \frac{9}{30}) = -(14 + \frac{14+9}{30}) = -(14 + \frac{23}{30}) = -14\frac{23}{30}$$

Ответ: 1) -15; 2) -1,12; 3) -2,14; 4) $$-7\frac{2}{7}$$; 5) $$-1\frac{17}{24}$$; 6) $$-14\frac{23}{30}$$