Given: ∠1 and ∠2 are supplementary, and ∠2 and ∠3 are supplementary.
prove: ∠1 ≅ ∠3
by definition of supplementary angles, m∠1 + m∠2 = (a) and m∠2 + m∠3 = (b).
then, m∠1 + m∠2 = m∠2 + m∠3 by the (c). subtract m∠2 from each side. you
get m∠1 = (d), or ∠1 ≅ (e).