NSum

NSum[f, {i, imin, imax}] gives a numerical approximation to the sum.

NSum[f, {i, imin, imax, di}] uses a step di in the sum.

Examples

Numerical sum:

NSum[1/n^2, {n, 1, Infinity}]
(* 1.64493 *)

Finite sum:

NSum[n, {n, 1, 100}]
(* 5050. *)

Please visit the official Wolfram Language Reference for more details.