Properties Alternate Angles Alternate angles are equal From the diagram above, angle DCB and angle OBA are alternate angles Hence angle DCB=OBA Sum of angles in triangle Sum of angles in a triangle add up to 180∘ A∘+B∘+C∘=180∘ From the diagram in the question, angle x∘+ angle OBA+ angle BAO=180∘ (Sum of angles in triangle) x∘+OBA∘+20∘=180∘ x∘=180∘−20∘−OBA∘ x∘=160∘−OBA∘
Sum of angles on a straight line x+y=180 (sum of angles on a straight line add up to 180) From the diagram in the question, 120+ angle DCB = 180 (Sum of angles on a straight line) 120+DCB=180 DCB=180−120 DCB=60
Angle DCB=OBA, alternate angles If angle DCB=60, then angle OBA is also 60 From x∘=160∘−OBA∘ OBA=60 x=160−60 x=100