The straight lines AB and CD intersect in O.

∴ ∠AOC + ∠AOD = 180° … (1) (Linear pair)

∠AOD + ∠BOD = 180° … (2) (Linear pair)

From (1) and (2), we get

∠AOC + ∠AOD = ∠AOD + ∠BOD

∠AOC = ∠BOD

Similarly, ∠BOC = ∠AOD

Thus, if two straight intersect each other, then the pair of vertically opposite angles are equal.