What is the answer of series of $n+(n-1)+(n-2)+\cdots.+1$

Question

I need a help regarding my question I searched a lot Somewhere I found the answer of the sequence is $\frac{n(n-1)}{2}.$

Answer 1

$S_n=1+2+\dots+(n-1)+n$

$S_n=n+(n-1)+\dots+2+1$

$2S_n=n(n+1)$, from where you get the desired formula.