A smokestack deposits soot on the ground with a concentration inversely proportional to the square of the distance from the stack. With two smokestacks miles apart, the concentration of the combined deposits on the line joining them, at a distance from one stack, is given by where and are positive constants which depend on the quantity of smoke each stack is emitting. If , find the point on the line joining the stacks where the concentration of the deposit is a minimum.

= mi