Question

1题：（4000-1-2-3-、、、-77-78=）2题（3000-2-4-6-、、、-78-80）

Answer 1

1.

4000-1-2-3-...-78

=4000-(1+2+3+...+78)

=4000-(1+78)/2 *78

=4000-3081

=919

2.

3000-2-4-6-...-78-80

=3000-(2+4+6+...+78+80)

=3000-(2+80)/2*[(80-2)/2+1]

=3000-1640

=1360