We have to prove ∠1=∠3 & ∠2=∠4
∠1+∠2=180∘ .......(i) [Linear pair]
⇒∠1=180∘−∠2
Again, ∠3+∠2=180∘ .......(ii) [Linear pair]
⇒∠3=180∘−∠2
Hence, ∠1=∠3 [from (i) and (ii)]
Similarly, we can prove that ∠2=∠4.