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.

$f(n)=\binom{n+2}{2}=\frac{(n+1)(n+2)}{2}$
If you have a sequence where the differences between terms is lineal then it is going to be a quadratic polynomial.
