Show the independence between two i.i.d random variables

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Let X,Y are independent random variables with equal distrbution (identically distributed). Show, that E(X| X + Y ) = E(Y |X + Y ) = (X + Y) / 2

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By symmetry, we have $E(X\mid X + Y) = E(Y \mid X+Y)$ (good exercise to formally justify this with the definition of conditional expectation).

Hence $X+Y = E(X+Y\mid X+Y) = E(X\mid X+Y) + E(Y\mid X+Y) = 2 E(X\mid X+Y)$

Thus $E(X \mid X+Y) = (X+Y)/2= E(Y \mid X+Y)$.