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Can anyone explain: Let $B$ be a Borel set and $B + a = \{ x + a : x \in B\}$. Why is $B + a$ a Borel set?

I think I have to use some good set principle but not sure how to complete the proof.

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    This amounts to showing that the family of invariant Borel sets forms a $\sigma$-algebra that contains all Borel sets.2012-10-10

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