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.