Statistics?

Algebra

The elements of set $S$ of real numbers with cardinality $n$ form an arithmetic progression with common difference $d$ . Express the population standard deviation of set $S$ in terms of $n$ and $d$ .

An example of set $S$ is $\text{{2, 5, 8, 11, 14, 17}}$ . It has cardinality 6, and its elements form an AP with common difference 3.