Proof: Substitute Two Variables Theorem

Let's prove the following theorem:

if the following are true:
  • a = x
  • b = y

then ab = xy

Proof:

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Given
1 a = x
2 b = y
Proof Table
# Claim Reason
1 ab = xb if a = x, then ab = xb
2 xb = xy if b = y, then xb = xy
3 ab = xy if ab = xb and xb = xy, then ab = xy
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