to view the calculus menu, then 
to select numerical integration.
The syntax is: fnInt( expression,variable
,startValue,endValue ).
The numerical integration option approximates the definite integral of expression with respect to variable from startValue to endValue.
tol is used to determine how accurate to calculate
the integral. The TI tries to keep the absolute error under tol.
A smaller value yields a more accurate integral but takes longer to compute.
It can be changed in the tolerance editor available by pressing
,
typically about 0.001.For more specific information and tips see Integration on the TI-how does it work?.