Consider the twice-differentiable utility function u(x1, x2) defined over the goods x1, x2. Defined an indifference curve for this function by = u(x1, x2) as the various combinations of (x1, x2) that yield a given level of utility . (a) (15 marks) Totally differentiate the expression expression for the slope of the indifference curve, der dx2 = . u(x1, x2), and find an (b) (10 marks) What property must we assume that the utility function has to imply that the slope of the indifference curve is negative?