In Fig. 6.16, if AB ∥ CD, then
(a) ∠2 = ∠3
(b) ∠1 = ∠4
(c) ∠4 = ∠1 + ∠2
(d) ∠1 + ∠2 = ∠3 + ∠4
By parallel lines property, sum of angles on transversal equal. So ∠1+∠2 = ∠3+∠4.