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A boy ‘A’ is running on a circular track...

A boy ‘A’ is running on a circular track of radius R. His friend, standing at a point O on the circumference of the track is throwing balls at speed `u = sqrt(gR)`. Balls are being thrown randomly in all possible directions. Find the length of the circumference of the circle on which the boy is completely safe from being hit by a ball.

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The correct Answer is:
`(4)/(3)piR`
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