Let the radius of the circle with center R be ‘r’ cm. Note: If two circles touch each other (internally or externally) then the line joining their centers will always pass though the point of contact. The circle with center R and the smaller circle with center O touch each other externally. Hence, OR = OS + SR = 1 + r ...(i) Also, OT must pass through R as the circle with center R and the larger circle with center O touch each other internally. Hence, OT = 3 = OR + RT = 1+ r + r=1+2r ...from (i) ⇒ r = 1 cm.