In an experiment in the civil engineering department of Virginia Polytechnic Institute and State University in 1988, a growth of certain type of algae in water was observed as a function of time and dosage of copper added into the water. The collected data are shown in Table 3.28. (a). Consider the following regression model yi = β0 + β1x1i + β2x2i + β12x1ix2i + εi Estimate the coefficients of the model, using multiple linear regression. (b). Test H0 : β12 = 0 versus H1 : β12 6=