Please Use R code.(a) Is model (8.6) a valid model for the data? Give reasons to support your answer. (b) What extra predictor term or terms would you recommend be added to model (8.6) in order to...

Please Use R code.(a) Is model (8.6) a valid model for the data? Give reasons to support your answer. (b) What extra predictor term or terms would you recommend be added to model (8.6) in order to improve it. Please give reasons to support each extra term. (c) Following your advice in (b), extra predictor terms were added to model (8.6) to form model (8.7). We shall denote these extra predictors as 11 2 4 fx fx ( ) and ( ) (so as not to give away the answer to (b)). Marginal model plots from model (8.7) are shown in Figure 8.19. Is model (8.7) a valid model for the data? Give reasons to support your answer. (d) Interpret the estimated coefficient of x3 in model (8.7).