I m pretty sure that it s fine if the rates are understood to be per year or some other finite period as is usually the case in economics After a year in the bank an amount of money M becomes M 1 i where i is the nominal interest rate You can also write this as M 1 r 1 p where r is the real interest rate and p is the rate of inflation Then you get the first equality for r It reduces to i p when p is small or if the rates are measured instantaneously something that probably only a mathematicians would do