Complete the paragraph proof. given: ∠1 and ∠2 are supplementary, and ∠2 and ∠3 are supplementary. prove: ∠1 ≅ ∠3 angle 1 is acute, angle 2 is obtuse, and angle three is acute. 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).