Use TREND function with nonadjacent cells
 

I am trying to use the TREND function with nonadjacent cells, but doesn't
works!
For example:
I have a column with diferents rows for months (cells C1:C12) and a total at
the end of the 2004 year (cell C13)
After that, I have more rows for months of the 2005 year (cells C14:C25)
and their total (cell C26)
When I try to get the trend of months
Oct'04-Nov'04-Dic'04-Jan'05-Feb'05-Mar'05 (last 6 months) I can't select the
range C10:C16 because the total cell (C13) is included. When I select the
range with nonadjacent cells, Excel show me an error.
Someone could help me? Thanks...