An article in Rubber Chemistry and Technology (Vol. 47, 1974, pp. 825€“836) describes an experiment that studies the relationship of the Mooney viscosity of rubber to several variables, including silica filler (parts per hundred) and oil filler (parts per hundred). Some of the data from this experiment are shown in Table 14E.2, where

(a) What type of experimental design has been used? Is it rotatable?
(b) Fit a quadratic model to these data. What values of x1 and x2 will maximize the Mooney viscosity?

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silica 60 15 oil-21 21.5 TABLE 14E.2 The Viscosity Experiment for Exercise 147 Coded Levels Ху 12 13.7 14.15 2.87 13 53 12.99 13.89 14.16 -14 14 0 0 0 0 0 14 12.90 0 0 0 0 0 13.75 13.66 13.86 13.63 13.74 0