urgent query

Please clear my these Doubts in a very simple way or say like ur teaching to a layman...

1 WHAT IS LOGIC BEHIND THE FORMULA OF EOQ ie EOQ= ROOT 2AB/CS

2 HOW EOQ = ROQ

3 WHAT IS LOGIC BEHIND THIS FORMULA OF REORDER LEVEL IE  REORDER LEVEL= MAXIMUM USAGE PER DAY*MAXIMUM LEAD TIME

AND ONE MORE DOUBT IF ANYONE IS IPCC STUDENT PLEASE CHECK AND TELL ME 

 IN PAGE NO 2.31 OF COSTING MODULE IPCC 

IN EQUATION 3 ie 1/2*Q*I WHY THEY HAVE TAKEN 1/2

I request everyone in caclubindia kindly help me to clear this doubt