# Question

Refer to the data of Exercise 13.41. Suppose that another variable was controlled, that the first four squares at each speed were treated with a .2 concentration of protective coating, and that the second four squares were treated with a .4 concentration of the same coating. Given that x1 de-notes the machine speed and x2 denotes the concentration of the protective coating, fit these models:

y = β0 + β1x1 + β2x12 + β3x2 + ɛ

y = β0 + β1x1 + β2x21 + β3x2 + β4x1x2 + β3x21x2 + ɛ

In exercise

y = β0 + β1x1 + β2x12 + β3x2 + ɛ

y = β0 + β1x1 + β2x21 + β3x2 + β4x1x2 + β3x21x2 + ɛ

In exercise

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