Margrt attaches a light bulb to the ceiling to illuminate a picture that hangs d meters below the ceiling. Find the distance x from the wall where the bulb provides maximum brightness, given that:

1. The brightness decreases inversely proportional to the square of the distance, f, from the light source.
2. The brightness on a surface is directly proportional to cos, where is the angle between the light ray and the vector perpendicular to the surface (Lambert's cosine law).