ln(y) = b0 + b1*ln(x1) + b2*ln(x2) + 0.5*b11*ln(x1)*ln(x1) + 0.5*b22*ln(x2)*ln(x2) + b12*ln(x1)*ln(x2)

dy/dx1 = [ d ln(y) / d ln(x) ] * [y/x]