Proof: Add Number to Both Sides

Let's prove the following theorem:

if a + 90 = 180, then a = 90

Proof:

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Given
1 a + 90 = 180
Proof Table
# Claim Reason
1 180 = a + 90 if a + 90 = 180, then 180 = a + 90
2 180 + (90 ⋅ (-1)) = a if 180 = a + 90, then 180 + (90 ⋅ (-1)) = a
3 a = 180 + (90 ⋅ (-1)) if 180 + (90 ⋅ (-1)) = a, then a = 180 + (90 ⋅ (-1))
4 a = 90 if a = 180 + (90 ⋅ (-1)), then a = 90

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