Trey Prompt #2

A.
1 = 1
2 = 3
3 = 6
4 = 10
5 = 15
6 = 21
7 = 28
8 = 36
9 = 45
10 = 55

B.
I am thinking that there is a pattern, you add the sum of what you got last time to the next number in number line.. I guess that is what you call it.
Example: 1+2=3 and then 3+3=6 and so on and so forth.

C.
I know that there is a formula but I cant seem to figure it out.

D.
I am positive that there is a formula but I am not sure that I can prove it correct.