answer all questions with explanation

\"A farmer is planning to fence and fertilize a rectangular plot. Let X and Y represents the width and height for the plot respectively and suppose that the fence he has in total of 20. He wants to at least have all the sides fenced, but don't need to use all the fences. He also found that this is a very irregular plot, the fertilizer needed by a certain area follows this joint density function of X and Y: xzy f (x. y) = ' 0. elsewhere 0