Work out an nth term formula for the triangular numbers 1,3,6,10,15,21,...
Thanks in advance.
Hello,
Please note the pattern :
$\displaystyle \begin{array}{|c||c|c} \text{Rank }(n) & a_n & \text{Difference with the previous term} \\ \hline
1 & 1 & \emptyset \\
2 & 3 & 2=n \\
3 & 6 & 3=n \\
4 & 10 & 4=n \\
5 & 15 & 5=n \end{array}$
So, what is $\displaystyle a_{n}-a_{n-1}$ ?