14.9 The article “Pulp Brightness Reversion: Influence of Residual Lignin on the Brightness Reversion of Bleached Sulfite and Kraft Pulps” (TAPPI [1964]: 653–662) proposed a quadratic regression model to describe the relationship between x 5 degree of delignification during the processing of wood pulp for paper and y 5 total chlorine content. Suppose that the population regression model is y 5 220 1 75x 2 4x2 1 e

a. Graph the regression function 220 1 75x 2 4x2 over x values between 2 and 12. (Hint: Substitute x 5 2, 4, 6, 8, 10, and 12 to find points on the graph, and connect them with a smooth curve.)

b. Would mean chlorine content be higher for a degree of delignification value of 8 or 10?

c. What is the change in mean chlorine content when the degree of delignification increases from 8 to 9? From 9 to 10?