Explanation:

$$\triangle$$ OPQ is a right angle triangle because $$\angle Q = 90\degree$$,
By Pythagoras,
$$(OQ)^2 + (PQ)^2 = (OP)^2$$
$$(OQ)^2 = (13)^2 - (12)^2$$
$$(OQ)^2 = 169 - 144$$
$$(OQ)^2 = 25$$
$$OQ = 5$$
$$\therefore$$ The radius of the circle is 5 cm