1. Micro and total differentiation review. (25 marks) Consider the twice-differentiable utility function u(X1, 12) defined over the goods X1, 2. Defined an indifference curve for this function by U - u($1, X2) as the various combinations of (X1, 12) that yield a given level of utility U. (a) (15 marks) Totally differentiate the expression U = u(X1, $2), and find an expression for the slope of the indifference curve, da da2 (b) (10 marks) What property must we assume that the utility function has to imply that the slope of the indifference curve is negative