№1 1) -3,9+(-1,9+3,9) 2) $$\frac{5}{21} - (\frac{11}{21} - \frac{4}{7})$$

Решение №1
1) -3,9 + (-1,9 + 3,9) = -3,9 + 2 = -1,9
\(\textbf{Ответ: -1,9}\)

2) $$\frac{5}{21} - (\frac{11}{21} - \frac{4}{7})$$ = $$\frac{5}{21} - (\frac{11}{21} - \frac{12}{21})$$ = $$\frac{5}{21} - (-\frac{1}{21})$$ = $$\frac{5}{21} + \frac{1}{21} = \frac{6}{21} = \frac{2}{7}$$
\(\textbf\){Ответ: $$\frac{2}{7}$$}