Capital ∑ is used for mathematical summation symbols, such as ∑Pi, where i= 1, 2, ..., t, which is the sum ...+angle of P 1+P2+. Lowercase σ is used for statistical standard deviation. Both the с of Cyrillic alphabet and the S of Latin alphabet are derived from Sigma. It also means summation, which means ∑ j =1+2+3+…+n.

∫

In calculus

Integration is the inverse operation of differentiation, that is, knowing the derivative function of a function and finding the original function in reverse. In application

In fact, the function of integral is not only that, but it is widely used in summation, that is, to find the area of curved polygon. This ingenious solution is determined by the special properties of integral.