Let us consider a circle centred at point O.
AB is a tangent drawn on this circle from point A.
OA = 5cm and AB = 4 cm
OB ⊥ AB (Radius ⊥ tangent at the point of contact)
Applying Pythagoras theorem in ΔABO, we obtain
AB2 + BO2 = OA2
42 + BO2 = 52
16 + BO2 = 25 BO2 = 9
BO = 3
Hence, the radius of the circle is 3 cm.
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