Theorem subsub4d

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	subsub4d	|- ( 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		subsub4	|- ( ( 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 ) ) )