The given sequence is the sum of two arithmetic sequences, 3n terms of
the first sequence and n terms of the second sequence:

1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 17 + 19 + 21 + 23 + ... + 3nth term
      -10         - 22           - 34           - 46 - ... -  nth term
----------------------------------------------------------------------
1 + 3 - 5 + 7 + 9 - 11 + 13 + 15 - 17 + 19 + 21 - 23 + ...     ?

The first sequence has 3n terms, but first we find the sum of n terms 



and since it has 3n terms, we replace n by 3n and get the sum of 3n terms,
 or  

The second sequence -10-22-34-46- ... has only n terms:




Adding the two sums:  

Answer: 
 
Edwin