Given that each of the angles AOC and AOB is a right angle. Show that BOC is a line.
        

Given: ∠ BOA = 90° and ∠ COA = 90°
To prove: BOC  is a line.
Proof: BOA and COA are adjacent angles.
                       ∠ BOA = 90°    (given)
                       ∠ COA = 90°      (given)
⇒ ∠ BOA + ∠ COA = 90°+ 90° = 180°         
⇒  BOA and COA form a linear and it have a common arm OA,
.^.. BOC is a line.