How To Use The Trapezoidal Rule In Microsoft Excel

The trapezoidal rule is used to approximate the integral of a function. The rule involves treating the area under a curve as a series of trapezoidal slices. Implementing this rule in Excel requires inputting a curve's independent and dependent values, setting integration limits, setting slice parameters and using a function to determine area.

Step 1

Input the curve you wish to analyze in an Excel spreadsheet. Put the independent values (i.e., x values) in the first column. Put the dependent values (i.e., f[x] values) in the second column.

Step 2

Determine the desired limits of integration. For example, if you want to find the area under a curve between x = 0 and x = 5, your limits of integration are 0 and 5.

Step 3

Delete any values outside the limits of integration in the first two columns of your table.

Step 4

Determine the desired number of trapezoidal slices. Divide this value by the range of your integration limits to get slice length. For example, if you want five slices between x = 0 and x = 5, your slice length will be one.

Step 5

Delete all independent values that are not either an integration limit or a multiple of the slice length. Delete all corresponding dependent values.

Step 6

Create a function in the top box of the third column: one-half the slice length times the sum of f[x] and f[x+1]. If your dependent values start in the first row and second column, type (1/2)(slice length)(B1+B2).

Step 7

Drag the bottom right of this function box down until the third column is one value short of the first two columns.

Step 8

Sum the values of the third column to get the approximate value of your integral.