Diagonal AC of a parallelogram ABCD bisects ∠A (see figure). Show that (i) it bisects ∠C also, (ii) ABCD is a rhombus -Maths 9th

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Answer :

(i) Here, ABCD is a parallelogram and diagonal AC bisects ∠A. ∴  ∠DAC=∠BAC      ---- ( 1 ) Now, AB∥DC and AC as traversal, ∴  ∠BAC=∠DCA          [ Alternate angles ]  --- ( 2 ) AD∥BC and AAC as traversal, ∴  ∠DAC=∠BCA         [ Alternate angles ]   --- ( 3 ) From ( 1 ), ( 2 ) and ( 3 ) ∠DAC=∠BAC=∠DCA=∠BCA ∴  ∠DCA=∠BCA Hence, AC bisects ∠C. (ii)  In △ABC, ⇒  ∠BAC=∠BCA      [ Proved in above ] ⇒  BC=AB      [ Sides opposite to equal angles are equal ]    --- ( 1 ) ⇒  Also, AB=CD and AD=BC     [ Opposite sides of parallelogram are equal ]       ---- ( 2 ) From ( 1 ) and ( 2 ), ⇒  AB=BC=CD=DA Hence, ABCD is a rhombus.

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