=+b. Fitting the complete second-order model gave the following results:

Predictor Coef SE Coef T P Constant -119.49 18.53 -6.45 0.000 x1 -0.1047 0.2839 -0.37 0.718 x2 28.678 3.625 7.91 0.000 x3 0.4074 0.1303 3.13 0.007 x4 0.2711 0.2606 1.04 0.316 x1sqd -0.000752 0.002110 -0.36 0.727 x2sqd -1.6452 0.2110 -7.80 0.000 x3sqd 0.0002121 0.0005275 0.40 0.694 x4sqd -0.015152 0.002110 -7.18 0.000 x1x2 0.02150 0.02687 0.80 0.437 x1x3 0.000550 0.001344 0.41 0.688 x1x4 -0.000800 0.002687 -0.30 0.770 x2x3 -0.05900 0.01344 -4.39 0.001 x2x4 0.03900 0.02687 1.45 0.169 x3x4 0.002725 0.001344 2.03 0.062