The diagonal AC of the parallelogram ABCD bisects ∠BAD. Prove that the diagonal AC also bisects ∠BCD.
Given ABCD is a parallelogram with diagonals AC and BD.
Also, AC bisects ∠BAD
So, ∠DAC=∠BAC …………..(1)
Since, CD||AB,
∠DCA=∠BAC (alternate interior angles)
And ∠BCA=∠DAC (alternate interior angles)
From (1),
∠DAC=∠BAC
⇒ ∠BCA=∠DCA
Hence, AC bisects ∠BCD