Proof: Cosine

Let's prove the following theorem:

Proof:

Proof Table

#	Claim	Reason
1	∠CBA is a right angle	if ∠ABC is a right angle, then ∠CBA is a right angle (Right Angle Reverse)
2	cosine of (m∠BAC) = (distance BA) / (distance CA)	if ∠CBA is a right angle, then cosine of (m∠BAC) = (distance BA) / (distance CA) (Definition of Cosine)

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