A chemical engineer is investigating how the amount of conversion of a product from a raw material (y) depends on reaction temperature (x1) and the reaction time (x2). He has developed the following regression models:

Both models have been built over the range 0.5 (a) What is the predicted value of conversion when x2 = 2? Repeat this calculation for x2 = 8. Draw a graph of the predicted values for both conversion models. Comment on the effect of the interaction term in model 2.
(b) Find the expected change in the mean conversion for a unit change in temperature x1 for model 1 when x2 = 5. Does this quantity depend on the specific value of reaction time selected? Why?
(c) Find the expected change in the mean conversion for a unit change in temperature x1 for model 2 when x2 = 5. Repeat this calculation for x2 = 2 and x2 = 8. Does the result depend on the value selected for x2? Why?

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= 100 + 2x, + 4x2 ý = 95 + 3x2 + 2xx2 + 1.5x,