An aeronautical engineering student carried out an experiment to study how y = lift drag ratio related to the variables x1 = position of a certain forward lifting surface relative to the main wing and x2 = tail placement relative to the main wing, obtaining the following data (Statistics for Engineering Problem Solving, p. 133):

a. Fitting the first-order model gives SSE = 5.18, whereas including x3 = x1x2 as a predictor results in SSE = 3.07. Calculate and interpret the coefficient of multiple determination for each model.
b. Carry out a test of model utility using α = .05 for each of the models described in part (a). Does either result surprise you?

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