proof this theorem

Q.  The length of tangents drawn from an external point to a circle are equal. 

To prove- the tangents drawn from an external point to a circle are equal in length. Proof- triangle PAO and triangle PBO PO =PO (common) angle PAO= angle PBO (90) AO =BO (radii) By RHS congruence rule Triangles are congruent PA = PB (CPCT) Hence proved