Consider a two-period economy with investment, as studied in class. The optimal invest- ment at t = 1 is characterized by the following optimality condition: A2f'(k2) = r+ 8. (1) Explain why the left-hand side is called 'marginal product of capital' (MPK) and the right-hand side is called 'user cost of capital', and why the optimality conditions equalizes the two.