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 = degree of delignification during the processing of wood pulp for paper and y = total chlorine content. Suppose that the population regression model is y= 5 220 + 75x - 4x2 + e.
a. Graph the regression function 220 + 75x = 4x2 over x values between 2 and 12. (Substitute x = 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?