Sigma, the symbol Σ, is used to denote a summation in math. A summation adds a set of numbers together. The sigma is used on a TI89 when calculating a summation. A TI89 is a calculator developed by TI, or Texas Instruments. A summation is written with the upper index above the sigma symbol and the lower index written below the sigma symbol. The expression is written after the sigma symbol.

Press "F3" and then press "4" on the TI89. You will see "Σ(" on the screen.

Enter in the expression you want to sum after "Σ(" then type ",". For example, enter "Σ(k^2,".

Specify the index of the summation. In the example, this would be "k", so this would be "Σ(k^2,k".

Enter the lower index and upper index separated by a "," in that order. At the end, press the ")" key and then press the "ENTER" key. In the example, this would be "Σ(k^2,k,1,20)" where 1 is the lower index and 20 is the upper index. This will give the sum of k^2 from 1 to 20 which is 2870.