Working with the full quadratic regression model and a three-dimensional graphics program, draw pictures of the regression surface (similar to Figure 17.1 on page 504) for various values of the...

Working with the full quadratic regression model

and a three-dimensional graphics program, draw pictures of the regression surface (similar to Figure 17.1 on page 504) for various values of the parameters β₁; ... ; β₅
and various values of the explanatory variables X₁
and X₂. You will derive a better impression of the flexibility of this model if you experiment freely, but the following suggestions may prove useful:

Try both positive and negative values of the parameters.

 Try cases in which there are both positive and negative Xs, as well as cases in which the Xs are all positive.

 Try setting some of the parameters to 0.