Formatting question

[SergeantMike]

This is more a of formatting question than a solving question. I have a formula published as

Note: pi is not the 3.14.... we are used to. I am confused about the missing variable at the top of the sigma and what exactly am I supposed to be doing with the elements in the parenthesis?

 

[Dr.Peterson]

The i under the sigma means "over all values of i"; presumably [MATH]n_i[/MATH] are values from a known dataset or sequence.

The parentheses represent the binomial coefficients, also written as [MATH]_nC_2[/MATH] and called combinations. You can look those up if you are not familiar with the concept.