∠ABC=x and ∠ADC=2x. Find ∠COD
180 – 3x
In △AOB and △DOC,AB||CD
Therefore, ∠ABO=∠DCO=x and ∠BAO=∠CDO=2x (alternate interior angles)
∠AOB=∠DOC (vertically opposite angles)
Therefore, △AOB∼△DOC by AA similarity
So, ∠AOB=∠DOC=180−3x (angle sum property)