Theorem 3eqtr3rd

Description: A deduction from three chained equalities. (Contributed by NM, 14-Jan-2006)

	Ref	Expression
Hypotheses	3eqtr3d.1	$$⊢\phi \to A=B$$
	3eqtr3d.2	$$⊢\phi \to A=C$$
	3eqtr3d.3	$$⊢\phi \to B=D$$
Assertion	3eqtr3rd	$$⊢\phi \to D=C$$

Proof

Step	Hyp	Ref	Expression
1		3eqtr3d.1	$$⊢\phi \to A=B$$
2		3eqtr3d.2	$$⊢\phi \to A=C$$
3		3eqtr3d.3	$$⊢\phi \to B=D$$
4	1 2	eqtr3d	$$⊢\phi \to B=C$$
5	3 4	eqtr3d	$$⊢\phi \to D=C$$