(1+2+3)+(2+3+4)+(3+4+5)+...+(38+39+40)+(39+40+41)

=6+9+12+...+117+120

=3x(2+3+...+39+40)

=3x819

=2457

Not plus more 42, because 42 = 40 + 2 

And 2 appear 2 times and 40 appear 2 times => There must be 3(2+3+...+40)

Hey Dao Trong Luan, if you see more strictly, you will see the number 4 appear 3 times, do the same with all the number left, then it must be 3 ( 2+3+...+40) 

=> The value of the sum is:

(1+2+3+39+40+41) x 39 / 2 = 2457

The value of (1 + 2 + 3) + (2 + 3 + 4) + (3 + 4 + 5) + … + (38 + 39 + 40) + (39 + 40 + 41) is:

1 + 2.(2 + 3 + ... + 40) + 41 = 42 + 1638 = 1680