Theorem subsubd

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

	Ref	Expression
Hypotheses	negidd.1	|- ( ph -> A e. CC )
	pncand.2	|- ( ph -> B e. CC )
	subaddd.3	|- ( ph -> C e. CC )
Assertion	subsubd	|- ( ph -> ( A - ( B - C ) ) = ( ( A - B ) + C ) )

Proof

Step	Hyp	Ref	Expression
1		negidd.1	|- ( ph -> A e. CC )
2		pncand.2	|- ( ph -> B e. CC )
3		subaddd.3	|- ( ph -> C e. CC )
4		subsub	|- ( ( A e. CC /\ B e. CC /\ C e. CC ) -> ( A - ( B - C ) ) = ( ( A - B ) + C ) )
5	1 2 3 4	syl3anc	|- ( ph -> ( A - ( B - C ) ) = ( ( A - B ) + C ) )