Proof: Angle180 90

Let's prove the following theorem:

if 180 = a + 90, then a = 90

Proof:

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

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