Proof: Neg Nine Plus Nine Plus X

Let's prove the following theorem:

(-9) + (9 + x) = x

Proof:

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Proof Table
# Claim Reason
1 ((-9) + 9) + x = x ((-9) + 9) + x = x
2 ((-9) + 9) + x = (-9) + (9 + x) ((-9) + 9) + x = (-9) + (9 + x)
3 (-9) + (9 + x) = x if ((-9) + 9) + x = x and ((-9) + 9) + x = (-9) + (9 + x), then (-9) + (9 + x) = x

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