Help with Math?

Question

I need to find out what Y is. Here is an example of how to do it.
Answer: Y=2X+3

X Y
1 5
2 7
3 9
4 11
5 13

OK got it? Now the problem I need help on is this:

X Y
1 1
2 3
3 6
4 10
5
6
10
What would the Y= ???????????????

Answer 1

Y is the sum of the integers greater than zero, up to and including Y. 1=1, 1+2=3, 1+2+3=6, 1+2+3+4=10, etc.

X Y
1 1
2 3
3 6
4 10
5 15
6 21
7 28
8 36
9 45
10 55

Another formula is Y=X*(X+1)/2.