Refer to Exercise 13.8.
a. Fit the model y = β0 + β1x1 + β2x2 + β3x12 + β4x22 + β5x1x2 + ɛ to the aphid data.
b. Compare the fit of the linear model from Exercise 13.8 to the fully quadratic model fit in part (a) of this exercise.
c. Use residual plots, tests of hypotheses, and other diagnostic statistics to identify possible additional terms to add to the model fit in part (a).
In exercise

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