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Usually [itex]r=\sqrt{x^2+y^2+z^2}[/itex] which is the distance from the origin to the field point...In this case you want the distance from the z-axis which is usually given as the cylindrical coordinate [itex]s=\sqrt{x^2+y^2}[/itex]...You can call that [itex]r[/itex] if you like, but you need to make sure you state that it is the distance from the axis, not the distance from the origin.