Suppose you want to develop a model that predicts the revenue per visit to a company's web site. The explanatory variables you are going to use are
x1: household income
x2: time spent on the web site
x3: gender
(a) Write a model that utilizes all three explanatory variables in an additive model with linear terms. Be sure to define any indicator variables.
(b) Write a quadratic model that only utilizes time spent on the website.
(c) Suppose you suspect there is interaction between household income and time spent on the web site. Write a model that incorporates this interaction term into the model from part (a).