Let, O be the centre and AB is tangent at B.
Given : OA = 5cm and AB = 4 Cm
We Know that the radius is Perpendicular to tangent.
Therefore, In ΔABO, OB ⊥AB.
In ΔABO, by pythagoras theorem,
AB² +BO² = OA²
⇒ 4² + BO² = 5²
⇒ 16 + BO² = 25
⇒ BO² = 9
⇒ BO = 3
⇒ Therefore, the radius of circle is 3 cm.