If one side of a cyclic quadrilateral is produced, then the exterior angle is equal to the interior opposite angle.

Explanation

Given: A cyclic quadrilateral `ABCD`. Side `AB` is produced to `E`
To prove: `/_CBE=/_ADC`
Proof : `/_ABC+/_ADC=180^@` .......... (i)
Sum of opposite pairs of angles in a cyclic quadrilateral is equal to `180^@`
But `/_ABC+/_CBE=180^@` ............. (ii)[`/_ABC` and `/_CBE` are linear pairs]
From (i) and (ii),
`/_ABC+/_ADC=/_ABC+/_CBE`
`/_ADC=/_CBE` or `/_CBE=/_ADC`
Hence proved.