Triangle Number Formula
Add a formula that creates the triangle number of a number. According to wikipedia(And many other websites) the formula is n+(+1), but I don't see how that would work in excel. My idea is something like =TRIANGLE(A1).
2 comments

Anonymous commented
You could use the COMBIN function, the 7th triangular number (28) is obtained using =COMBIN(8,2), so to find the nth triangular number use =COMBIN(n+1,2)

Roy commented
The formula is just a sum of a sequence formula. So it is the number of items ("n" let's say) divided by 2 and that multiplied by the sum of the first number in the sequence ("1" for triangular numbers) and the last one (the number of items ("n" as I called it above) or better name: the number you want the triangular number for (still "n")):
Triangular number = (n/2) * (1 + n)
So in Excel, let's say you put the number you want a triangular number for into cell A1. Your formular would then be:
=( A1 / 2 ) * ( 1 + A1)
Hope that helps. My bet is they won't ever add this one because it would be a fairly rarely useful function, fairly specialized in its uses.
However, if they added a broader function, maybe called SEQUENCE() or SUMSEQUENCE(), some variation that, it might be useful to a lot more purposes and would completely fit your needs as well.
It could have three arguments (inputs): the ending number, the sequence's starting number (so the sequence could begin at any old number instead of at "1" — broader appeal, more chance of it happening), and the difference between each number in the sequence (so you could sum up sequences like 3,6,9,12... not just 3,4,5,6 type sequences). Broader appeal. And it could be set so if the 2nd and 3rd arguments are not there, it starts at "1" and goes up by "1" so for your use, you'd just have to give it the number you want a triangular number for.
The broader the appeal, the more likely you might be to get it! Good luck!