Theorem 3eqtr2i

Description: An inference from three chained equalities. (Contributed by NM, 3-Aug-2006)

	Ref	Expression
Hypotheses	3eqtr2i.1	|- A = B
	3eqtr2i.2	|- C = B
	3eqtr2i.3	|- C = D
Assertion	3eqtr2i	|- A = D

Proof

Step	Hyp	Ref	Expression
1		3eqtr2i.1	|- A = B
2		3eqtr2i.2	|- C = B
3		3eqtr2i.3	|- C = D
4	1 2	eqtr4i	|- A = C
5	4 3	eqtri	|- A = D