For a demand function u (x, y) = xy, show the demand
functions for good x and good y. (Remember that
MRS = (du/dx) / (du/dy) = px / py in
the point of interest, the tangency point of budget line and
indifference curve. The budget condition is given by
pxx + pyy = m)
for u (x, y) = x1/3 y2/3