Also see part 2 of 2 for the calculation method
If you use a strait line of best fit you can not be precise enough to satisfactorily predict values.
Triangular numbers are constructed as in the diagram.
You add up all the dots from the first one down to whichever point you wish to stop.
See part 1 of 2 first before reading this one.
The sequence is
Notice that the last value in the sum is the term number or line number.
Consider any one of the rows
Let the first value be
Let the last value be
Then the sum of any row is
Changing the order we have:
Compare this to the standardised equation of a quadratic