Theorem subsubd

Description: Law for double subtraction. (Contributed by Mario Carneiro, 27-May-2016)

	Ref	Expression
Hypotheses	negidd.1	⊢ ( 𝜑 → 𝐴 ∈ ℂ )
	pncand.2	⊢ ( 𝜑 → 𝐵 ∈ ℂ )
	subaddd.3	⊢ ( 𝜑 → 𝐶 ∈ ℂ )
Assertion	subsubd	⊢ ( 𝜑 → ( 𝐴 − ( 𝐵 − 𝐶 ) ) = ( ( 𝐴 − 𝐵 ) + 𝐶 ) )

Proof

Step	Hyp	Ref	Expression
1		negidd.1	⊢ ( 𝜑 → 𝐴 ∈ ℂ )
2		pncand.2	⊢ ( 𝜑 → 𝐵 ∈ ℂ )
3		subaddd.3	⊢ ( 𝜑 → 𝐶 ∈ ℂ )
4		subsub	⊢ ( ( 𝐴 ∈ ℂ ∧ 𝐵 ∈ ℂ ∧ 𝐶 ∈ ℂ ) → ( 𝐴 − ( 𝐵 − 𝐶 ) ) = ( ( 𝐴 − 𝐵 ) + 𝐶 ) )
5	1 2 3 4	syl3anc	⊢ ( 𝜑 → ( 𝐴 − ( 𝐵 − 𝐶 ) ) = ( ( 𝐴 − 𝐵 ) + 𝐶 ) )