Refer to the data of Exercise 13.44.
a. Can we test for lack of fit for the following model?
y = β0 + β1x1 + β2x12 + β3x2 + β4x22 + β5x1x2 + β6x1x22 + β7x12 x2 + β8x12 x22 + ɛ
b. Write the complete model for the sample data. Note that if there was replication at one or more design points, the number of degrees of freedom for SSLack would be identical to the difference between the number of parameters in the complete model and the number of parameters in the model of part (a).