Consider the first-order model equation in three quantitative independent variables
E(y) = 1 + 2x1 + x2 - 3x3
a. Graph the relationship between y and x1 for x2 = 1 and x3 = 3.
b. Repeat part a for x2 = -1 and x3 = 1.
c. How do the graphed lines in parts a and b relate to each other? What is the slope of each line?
d. If a linear model is first-order in three independent variables, what type of geometric relationship will you obtain when E(y) is graphed as a function of one of the independent variables for various combinations of values of the other independent variables?