Prove that:

> 
> If $ \alpha \equiv \beta $ and $ \beta \equiv \gamma $ then $ \alpha \equiv \gamma $
> 
> 
> 

I know that:

$ \alpha \equiv \beta $ if and only if $ \alpha \leftrightarrow \beta $ is a tautology.

Should I use this theorem to prove the semantic equivalence, or should I go another way?