Refer to the Geographical Analysis (Jan. 2007) study that demonstrated the use of satellite image maps for estimating urban population, presented in Exercise 12.18 (p. 670). A first-order model for census block population density (y) was fit as a function of the proportion of a block with low-density residential areas (x1) and the proportion of a block with high-density residential areas (x2). Now consider a complete second-order model for y.
a. Write the equation of the model.
b. Identify the terms in the model that allow for curvilinear relationships.