1. ## arithmetic sequences formula

So I have this problem. I've got an=a1+2(n-1) but in the answer they have an=n(n-1)
And I'm kind of confused.

2. ## Re: arithmetic sequences formula

according to the recursive formula

$\displaystyle a_{n+1}-a_n=2n$

$\displaystyle a_n-a_{n-1}=2(n-1)$

$\displaystyle a_{n-1}-a_{n-2}=2(n-2)$

etc.

so $\displaystyle a_n-a_1=2(n-1)$ is not correct

3. ## Re: arithmetic sequences formula

Originally Posted by nyanyona

So I have this problem. I've got an=a1+2(n-1) but in the answer they have an=n(n-1)
And I'm kind of confused.
Look HERE for the solution.

4. ## Re: arithmetic sequences formula

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

$\begin{matrix} 1 & 2 & 3 & 4 & 5& ... &n\\ 0 & 2 & 6 & 12 & 20 & ...\\ 0\cdot1&1\cdot2&2\cdot3&3\cdot4&4\cdot5&...&(n-1)n \end{matrix}$