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A bus travels between two stops, A and B. The bus starts from rest at A and accelerates at a constant rate of 
until it reaches a speed of 
. It then travels at this constant speed before decelerating at a constant rate of 
, coming to rest at B. The total time for the journey is 
. Find an expression, in terms of 
, for the length of time 
that the bus is travelling with constant speed.
