申論題

10. The 72 rule says that money invested at 6% per year doubles in approximately 12 years. Also, money invested at 8 % per ycar doubles in approximately 9 years. Suppose that the number of years n is required for an investment at interest rate r to double in valuc, and i is the interest rate percentage (that is, i =100r). Using In2 = 0.693 and the approximation valid for small , show that it holds .