Data: OB is the radius, BA is the tangent.
∴ ∠OBA = 90°, OA = 5 cm. (data)
Length of tangent, AB = 4 cm.
∴ Radius, OB = ?
In ⊥∆OBA, ∠OBA = 90°
∴ OB2 + BA2 = OA2. OB2 + (4)2 = (5)2
OB2 + 16 = 25
OB2 = 25 – 16
OB2 = 9
∴ OB = 3 cm.
∴ Radius of circle, OB = 3cm.