zero, and is represented by the equation 0=MRS=Px(?X)+Py(?Y), where MU=Marginal Utility.
zero, and is represented by the equation 0=Px(?X)+Py(?Y), where MU=Marginal Utility.
positive, and is represented by the equation ?Satisfaction=MUx(?X)+MUy(?Y), where MU=Marginal Utility.
When the change in one good is positive, there must be a change in the opposite direction so that MUx(?X)= -MUy(?Y).