Conditional expectancy proof

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In the formal definition of conditional expectancy i have found this identity:
For any $A∈F$ we have $∫_A E(X|F) dP=∫_A Xd P$, that is $X=E(X|F)$, where X is a random variable
but not the proof for it, when i try to do it by myself i get that the conditional expectancy equals $E(X)$ , what am i doing wrong?