Math question: What does the symbol ∑ mean up and down?

This is a summation symbol.

The formula means

From n 1 to ni.

A symbol commonly used in mathematics, mainly used to find the sum of polynomials, expressed by ∑.

The small characters below ∑, such as i= 1, indicate that the sum starts from i= 1.

The little words above, such as n, mean add up to n.

For example, i= 1 is written below, N is written above, and Xi is written behind (I is the lower corner mark).

It means starting from x 1+x2+ ... +xn.

For example, i=5 is written below, N is written above, and Xi is written behind (I is the lower corner mark).

It means from x5+x6+…+xn.