Answer:
- Reason 2)
- The transitive property of congruence
Explanation:
There are several sort of "obvious" properties of equality/congruence. Keeping them sorted out can be a bit of a trick.
A = A . . . . reflexive property
A = B ⇔ B = A . . . . symmetric property (erroneously cited in the proof)
A = B, B = C ⇒ A = C . . . . transitive property (the one needed in the proof)
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Not to confuse the issue, but there is also the "substitution property of equality" that says if X = Y, then X can be substituted for Y or vice versa. In the end, the transitive property is an application of this, where we're substituting for B in the second equation, or for A in the first equation.
The A=B=C form of Statement 1) in the proof is what gives you the clue it is the transitive property that is wanted here.