I don't understand this, please explain.

Question: (partial molar enthalpy H1 and H2)

b) show the limit check

c) show the summability check

d) show the G/D check

note: a1,a2,b1,b2 are constants

For a particular binary liquid solution at constant T and P, the molar enthalpies of mixtures are represented by the equation H=x1(a1+b1x1)+x2(a2+b2x2)