a guest Sep 23rd, 2016 172 Never
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- Let x, y be given. We show that x + y = y + x.
- Observe 0 = (x + y) + (x + y). Add (y + x) from the left on both sides to get y + x = (y + x) + (x + y) + (x + y)
- = (y + (x + x) + y) + (x + y) = (y + y) + (x + y) = x + y. Q.E.D.
- Let x, y be given such that 0 = x + y. Add x from the left on both sides to get x = (x + x) + y = y. Q.E.D.
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