It is clear that µt = A. Let x,y ∈ X. Case(i): If x,y ∈ A then x + y and x − y ∈ A, Hence µ(x) = µ(y) = µ(x + y) = t and µ(x − y) = t. Therefore µ(x+ y) = t = t∧ t ≥ µ(x)∧ µ(y) and µ(x−y) = t = t∧ t ≥ µ(x)∧ µ(y). Case(ii): If x,y / ∈ A then µ(x) = µ(y) = 0. Then µ(x + y) ≥ 0 = 0 ∧ 0 = µ(x) ∧ µ(y) µ(x − y) ≥ 0 = 0 ∧ 0 = µ(x) ∧ µ(y).