# Homework Help: Find a formula for 1, 3, 6, 10, 15, 21,

1. Jul 18, 2010

### annoymage

1. The problem statement, all variables and given/known data

find the formula for

1,3,6,10,15,21,...

2. Relevant equations

n/a

3. The attempt at a solution

i only can find n>=3

Tn = 3 + $$\sum$$ i ; i=3 to n

help T_T

2. Jul 18, 2010

### mg0stisha

Re: Series

For one thing, the numbers in the way you listed them are called a sequence. Do you see a pattern among those numbers? That may help you.

Last edited: Jul 18, 2010
3. Jul 18, 2010

### annoymage

Re: Series

yea lol, it's sequence ;P sorry, and yea, i saw the pattern, but only start from 3 T_T

4. Jul 18, 2010

### HallsofIvy

Re: Series

But 3= 1+ 2 and 1= 0+ 1 so you can say it is $T_n= \sum_{i= 1}^n i$ That's a well known sum with a well known formula. Look up "triangular numbers".

5. Jul 18, 2010

### annoymage

Re: Series

thanks, how come i didn't realised it's $T_n= \sum_{i= 1}^n i$ instead of Tn = 3 + sum(i) ; i=3 to n.

thank you :D

6. Jul 18, 2010

### gomunkul51

Re: Series

There is no sigma notation in sequences, a sequence is written as:

$$\left \{a_{n} \right \}_{n=0}^{N}$$

as to your sequence it could be defined by a recursive formula:

$$a_{n+1}=a_{n}+n+2$$

$$a_{0}=1$$