Please attach the Prime worksheet. That will help us answer your question in the best way.
You should also know that this part of the Mathcad/Prime forum is mainly for installation questions.
Next time you should post here: https://community.ptc.com/t5/PTC-Mathcad/bd-p/PTCMathcad
This function calculates the area under the curve defined by x and y data.It uses the triangle rule:
See attached Prime worksheet.