# Math Help - Quadratic Sequences!

Work out an nth term formula for the triangular numbers 1,3,6,10,15,21,...

2. Hello,
Originally Posted by suhaster
Work out an nth term formula for the triangular numbers 1,3,6,10,15,21,...

$\begin{array}{|c||c|c} \text{Rank }(n) & a_n & \text{Difference with the previous term} \\ \hline
1 & 1 & \emptyset \\
2 & 3 & 2=n \\
3 & 6 & 3=n \\
4 & 10 & 4=n \\
5 & 15 & 5=n \end{array}$

So, what is $a_{n}-a_{n-1}$ ?

3. $T_n = \sum\limits_{k = 1}^n k$

4. Sorry to both of u but im not understanding what ur trying to say, can u tell me the answer and with it the explanation plz thats the best way of learning it for me. tar