dummy variable in FO

From: "Li Yi" <liyi.cn@xxxxxxxxx>
Date: 20 May 2006 00:10:38 -0700

Suppose x does not occur free in phi.
Show that

|= (exists x phi) <-> phi
and
|= (forall x phi) <-> phi

I don't really know how to do this. Thank you.

.

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