Γ and Σ are formula sets. Σ is satisfieable., and for every formula θ applies θ $\displaystyle \in$ Γ or $\displaystyle \neg$θ $\displaystyle \in$ Γ. Proof Γ $\displaystyle \subseteq$ Σ => Γ = Σ

I dont know how to proof....

Every help would be appreciated