For this solution,is this formula dGM$/$dxb$=0$ always true in the situation equilibrium like monotectic eutectic $,$even when xb is not equal to $0\mathrm{.}5?$

$\frac{\text{d}\text{e}\text{l}{G}^M}{del{x}_B}=RT[ln\frac{x_B}{x_A}]+(1-2x_B)$