Step-by-step explanation:
If ∠1 and ∠2 are supplementary, then m∠1 + m∠2 = 180° (1).
If ∠2 and ∠3 are supplementary, then m∠2 + m∠3 = 180° (2).
From (1) we have
m∠1 + m∠2 = 180° /subtract m∠1 from both sides
m∠2 = 180° - m∠1
substitute it to (2)
(180° - m∠1) + m∠3 = 180°
180° - m∠1 + m∠3 = 180° /subtract 180° from both sides
- m∠1 + m∠3 = 0 /add m∠1 to both sides
m∠3 = m∠1 ⇒ m∠1 = m∠3 ⇒ ∠1 ≅ ∠3
