13.18 Suppose E(Y) is related to two predictor variables x and x2 by the equation E(Y) 3+x 2x2 + x1x2

a. Graph the relationship between E(Y) and x, when x20. Repeat for x2 = 2 and for x2 = -2.

b. Repeat the instructions of part a for the model E(Y)=3+x-2x2

c. Note that the equation for part a is exactly the same as the equation in part b except that we have added the term xx2. How does the addition of the xx term affect the graphs of the three lines?

d. What flexibility is added to the first-order model E(Y) Bo+B+B2x2 by the addition of the term 3x12, using the model E(y) B2x2 + B3x1x2? Bo+Bx +