If ${\mathrm{x}}_1$ and ${\mathrm{x}}_2$ represent the mole fractions of a component A in the vapour phase and liquid mixture respectively and ${\mathrm{p}}_{\mathrm{A}}^{\mathrm{o}}$ and ${\mathrm{p}}_{\mathrm{B}}^{\mathrm{o}}$ represent vapour pressure of pure A and B, then total vapour pressure of liquid mixture is