Q. 214.8( 5 Votes )

m and n are two plan

Answer :

Draw CN ⊥ n and BM ⊥ m. Since BM ⊥ m, CN ⊥ n and m || n. 
Therefore CN || BM. Now according to laws of reflection, we know that angle of incidence is equal to angle of reflection
    ... At B, ∠1 = ∠2 ….(i)
and at C, ∠3 = ∠4 ….(ii)
Since CN || BN with transversal BC,     \\\\ ∠2 = ∠3 ….(Alternate interior angles)       Þ 2 ∠2 = 2 ∠3
       Þ ∠2 +∠2 = ∠3 + ∠3
       Þ ∠1 + ∠2 = ∠3 + ∠4
    ⇒ ∠ABC = ∠BCD
... AB || CD

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