Given: l || m
To Prove: ∠1 + ∠2 - ∠3 = 180°
Construction: Through C, draw CF || l || m
Proof: ∵ l || CF by construction and a transversal BC intersects them
∴ ∠1 + ∠FCB = 180°
| ∵ Sum of consecutive interior angles on the same side of a transversal is 180°
⇒ ∠1 + ∠2 - ∠FCD = 180° ...(1)
But ∠FCD = ∠3 ...(2)
| Alternate interior angles
∴ From (1) and (2),
∠1 + ∠2 - ∠3 = 180°