Here, length of tangent (AB) = 4 cm
OA = 5 cm
Find : radius (r) = ?
∴ OB ⊥ AB(line through centre to the point of contact is perpendicular)
Now, in right ΔOAB,
OA2 = OB2 + A B2 (Using Pythagoras theorem]
⇒ (5)2 = r2 + 42
⇒ 25 = r2 + 16
⇒ r2 = 25 – 16
⇒ r2 = 9
⇒ r = 3 cm
Hence, radius of the circle is 3 cm.