$D_{12}=ax_T^3+bx_T^2+cx_T$

$D_{12}=(x_2D_1+x_1D_2)\left[1+\frac{d\mathrm{ln}\gamma_1}{d\mathrm{ln}x_1}\right]$

$D_{12}=(x_2D_1+2x_1D_2)\left[1+\frac{d\mathrm{ln}\gamma_1}{d\mathrm{ln}x_1}\right]^{\alpha}$