Consider the regression model Yi = b0 + b1X1i + b2X2i + b3(X1i * X2i) + ui. Use Key Concept 8.1 to show: a. YX1 = b1 + b3X2 (effect of change in X1, holding X2 constant). b. YX2 = b2 + b3X1 (effect of change in X2, holding X1 constant). c. If X1 changes by X1 and X2 changes by X2, then Y = (b1 + b3X2)X1 + (b2 + b3X1)X2 + b3X1X2.