Two Angles are Supplementary when they add up to 180°.
m∠2 + m∠1 = 180° (*)
If ∠1 ≅ ∠3, then m∠1 = m∠3.
Substitute to the (*)
m∠2 + m∠3 = 180°
Therefore ∠2 and ∠3 are supplementary.
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