Suguru Region Sum Lines math question (Puzzle #15 by Phistomefel (Fog of War))

[macend]

In Puzzle #15 by Phistomefel (Fog of War) there is a step where I was stuck without using help. The help states: "by now it should be clear that full line lenghts are triangular numbers, and their segments are factors of them."
We know at this point that one segment of the line is 2 cell long and can contain 1/5/7/9. We don't know how long the rest of the line is.
So the help continues: "Considering the sum of these two as a factor of the total sum of the line only allows for the possibility of them adding up to 12, with the line summing to 36."
I lost on this part. Ho do we know it is 12 and why should it be 36 (an 8-cell line) and not only24?

[Maul]

If you add up the digits 1 to 6, they sum to 21, and if you add up the digits 1 to 7, they sum to 28.

[macend]

Ah, it's so easy after somebody tells me the answer. :) Thank you very much.