Question 5 ABCD is a quadrilateral in which AB = BC and AD = CD. Show that BD bisects both the angles ABC and ADC.
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Solution
AB = BC (given) ∴∠2=∠1 ...(i) [angles opposite to equal sides are equal] and AD = CD [given] ⇒∠4=∠3 ...(ii) [angles opposite to equal sides are equal] On adding Eqs. (i) and (ii), we get ∠2+∠4=∠1+∠3⇒∠BCD=∠BAD...(iii)InΔBADandΔBCD, AB = BC [given] ∠BAD=∠BCD[fromEq.(iii)] and AD = CD [given] ∴ΔBAD≅ΔBCD[bySAScongruencerule] Hence, ∠ABD=∠CBDand∠ADB=∠CDB i.e., BD bisects the angles ABC and ADC.