Theorem abssubd

Description: Swapping order of subtraction doesn't change the absolute value. Example of Apostol p. 363. (Contributed by Mario Carneiro, 29-May-2016)

	Ref	Expression
Hypotheses	abscld.1	$$⊢\phi \to A\in ℂ$$
	abssubd.2	$$⊢\phi \to B\in ℂ$$
Assertion	abssubd	$$⊢\phi \to \left|A-B\right|=\left|B-A\right|$$

Proof

Step	Hyp	Ref	Expression
1		abscld.1	$$⊢\phi \to A\in ℂ$$
2		abssubd.2	$$⊢\phi \to B\in ℂ$$
3		abssub	$$⊢\left(A\in ℂ\wedge B\in ℂ\right)\to \left|A-B\right|=\left|B-A\right|$$
4	1 2 3	syl2anc	$$⊢\phi \to \left|A-B\right|=\left|B-A\right|$$