When the x1’s, x2’s, . . . , and/or the xk’s are equally spaced, the calculation of the β^’s can be simplified by using the coding suggested in Exercise 14.15. Rework Exercise 14.80 coding the x1-values – 1, 0, and 1 and the x2-values –1 and 1. (For the coded x1’s and x2’s, call them z1’s and z2’s, we have not only ∑z1 = 0 and ∑z2 = 0, but also ∑z1z2 = 0.)