Given: PAQ and RBS be two parallel tangents to a circle with centre O.
To Prove: AOB is a straight line(joining two tangents) passing through center O..
Proof
Join OAandOB. Draw OCPQ.
Now, PAparallelCO
∠PAO+∠COA=180
[Sum of co-interior angle is 180]
90+∠COA=180[PAO=90]
∠COA=90
Similarly, ∠COB=90
∠COA+∠COB=90+90=180
Hence, AOB is a straight line passing through O