Theorem 3eqtr4i

Description: An inference from three chained equalities. (Contributed by NM, 26-May-1993) (Proof shortened by Andrew Salmon, 25-May-2011)

	Ref	Expression
Hypotheses	3eqtr4i.1	|- A = B
	3eqtr4i.2	|- C = A
	3eqtr4i.3	|- D = B
Assertion	3eqtr4i	|- C = D

Proof

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