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## Homework Statement

Let H be a subset of a group G. Prove: H does not equal the empty set and a,b are contained in H, which implies ab^(-1) is contained in H, which implies H is a subgroup.

Let G be a group. Let Ha = {x is contained in G | ax = xa }. Prove Ha is a subgroup of G

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## Homework Equations

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## The Attempt at a Solution

I looked at these and had issues figuring out where to start. If someone could give me somewhat of an idea of a starting point I could probably work it all out. I generally find this to be the biggest predicament for me: finding out where to start.