Finding expression for pattern This seems easy....
Pattern:
3, 6, 10, 15, 21....
How do I find expression for this?
It looked easy, but I couldn't find it.
I tried graphing it, but it turned out that it is not quadratic...
Thanks.

$$3\quad 3+\color{red}3=6\quad 6+\color{red}4=10\quad 10+\color{red}5=15\quad 15+\color{red}6=21\quad $$
