Here we use Excel to evaluate the integral of x2 from zero to an arbitrary upper limit, a. As shown in Figure 1, three columns were used.
Note that a picture is inserted as the header for the third column.
As shown in Figure 1, cell C6 has been named dx. Since cell C6 is selected, the name appears in a box in the upper left of the picture. We can now address the cell as dx rather than C6. The address dx is an absolute address, similar to $C$6. It will not change when we drag the rows down. Notice the error. For an upper limit of 1, the value of the integral should be 1/3 = 0.33333. Excel shows a value of 0.335. This error can be reduced by reducing dx. This will increase the number of rows required, but the accuracy will increase. For the trapezoidal rule, the error varies inversely as the square of the number of points.
Figure 3   To toggle between formulas and values, use; Formulas > Formula Auditing > Show Formulas