1)

[tex] - 6 + 9 + 6 = 9 \\ - 7 + 9 + ( - 2) = 2 - 2 = 0 \\ - 5 + 31 + 5 + ( - 31) = 31 - 31 = 0 \\ - 27 + 5 + ( - 4) + 27 = 5 - 4 = 1 \\ - 7 + 11 + 8 - 12 = 19 - 19 = 0 \\ 3 + 7 + ( - 3) + ( - 1) = 3 + 7 - 3 - 1 = 7 - 1 = 6[/tex]

2)

[tex]5 + (m - n) = 5 + m - n \\ 7 - ( - a + b) = 7 + a - b \\ - 3 - (m + n) = - 3 - m - n \\ 5 - (a - b + c) = 5 - a + b - c[/tex]

3)

[tex]0.9 - (a + 3.7) = 0.9 - a - 3.7 = - 2.8 - a \\ b + (7.8 - b - 1.9) = b + 7.8 - b - 1.9 = 5.9 \\ - (4 - a + 0.2) - 11.8 = - 4 + a - 0.2 - 11.8 = a - 16 \\ - (a + 5.1) - ( - a - 4.9) = - a - 5.1 + a + 4.9 = - 0.2[/tex]

4)

[tex]a + b - (b - c) + d = a + b - b + c + d = a + c + d = - 3.1 - 0.2 - 5 = - 8.3[/tex]