# on the TI-89

Finding a mathematical equation to represent real-world data is the first step in applying the power of mathematics to real-world problems. This process is called mathematical modeling. Here is a short tutorial on mathematical modeling on the TI-89.

## Fitting an Equation to the Data:

 To fit an equation to your data, press (Calculate). In the Calculate Dialog that opens, set: Calculation Type to "LinReg" x to "c1" y to "c2" Store RegEQ to y1(x) then press . Here are the results of the linear regression calculation on the data from this example. Correlation ("corr") and "R2" are measures of how closely the data fit the given line. For our purposes, a correlation close to 1 or -1 means "good fit." (The y-intercept shown here differs from the one in the text (which is -2707.25) since my x-values are adjusted from the ones in the text.) You can press (Y=) to check that, sure enough, the regression equation has been saved to y1. You can go ahead and graph this line, but it won't show your data points. To get back to your data, press .

## Graphing the Data:

 To set up a data plot: if you are currently on the Data/Matrix screen, press (Plot Setup). if you are on the "Y=" screen, scroll up to "Plots", highlight "Plot 1:", and press . Select (Define). In the Define Plot Dialog, set: Plot Type to "Scatter" Mark to "Box" x to "c1" y to "c2" and press . Press (Y=) to go to the Y= Editor. Then press (Zoom) and select "9:ZoomData" to graph the data points and the regression line. Compare this graph to Fig. 6 on p. 28 of the Stewart text.

last update April 14, 2008 by JL Stanbrough