A gardener is watering his plants on sloppy land inclined at 20° as shown in the figure below. The water is leaving the pipe at an angle of θ° with respect to the slope. Determine the angle at which the water should leave the pipe so that it covers the maximum distance 'd'.

Hint: use the following expression d/dθ(A•cosθ•sinθ - B•sin²θ) = A•cos(2θ) - B•sin(2θ)