Below is a two-column proof incorrectly proving that the three angles of ΔPQR add up to 180°: Draw line ZY parallel to segment PQ Construction m∠ZRP + m∠PRQ + m∠QRY = m∠ZRY Angle Addition Postulate ∠ZRP ≅ ∠RPQ Alternate Interior Angles Theorem ∠QRY ≅ ∠PQR Alternate Interior Angles Theorem m∠RPQ + m∠PRQ + m∠PQR = m∠ZRY Substitution m∠ZRY = 180° Definition of Supplementary Angles m∠RPQ + m∠PRQ + m∠PQR = 180° Substitution