13: \(\left(\right. - 15 \left.\right) + 8 + 7\)

\(= \left(\right. - 15 \left.\right) + \left(\right. 8 + 7 \left.\right)\)

=-15+15

=0

14: \(\left(\right. - 8 \left.\right) + 2 + 6\)

\(= \left(\right. - 8 \left.\right) + \left(\right. 2 + 6 \left.\right)\)

=-8+8

=0

15: \(\left(\right. - 1 \left.\right) + 3 - 2\)

\(= \left(\right. - 1 - 2 \left.\right) + 3\)

=-3+3

=0

16: \(25 - 8 - 7\)

\(= 25 - \left(\right. 8 + 7 \left.\right)\)

=25-15

=10

17: \(8 - 2 - 6\)

\(= 8 - \left(\right. 2 + 6 \left.\right)\)

=8-8

=0

18: \(\left(\right. - 12 \left.\right) - 3 + 15\)

\(= \left(\right. - 12 - 3 \left.\right) + 15\)

=-15+15

=30

1)

 25 x 7 + 3 x (50-25) x (60 - 60)

=175 +3 x 25 x 0

=175

2)

\(\frac{1}{2} . \frac{3}{4} . \frac{4}{5} . \frac{1}{3} = \frac{1.3.4.1}{2.4.5.3} = \frac{3.4}{3.4.2.5} = \frac{1}{2.5} = \frac{1}{10}\)

1995.1994−1​=1993.1995+19941995.(1993+1)−1​

  =\(\frac{1995.1993 + 1995 - 1}{1993.1995 + 1994} = \frac{1995.1993 + 1994}{1993.1995 + 1994} = 1\)