Four person K, L, M, N are initially at the four corners of a square of side d. Each person now moves with a uniform speed v in such a way that K always moves directly towards L, L directly towards M, M directly towards N, and N directly towards K. The four person will meet at a time _____________.

A particle moves in a circle of radius R. In half the period of revolution its displacement is ________ and distance covered is _________.