Rate of interest is said a parameter of growth of the capital. In the paper a way is presented from the idea of a parameter to the idea of geometrical and algebraic object. If the economic process is said a real function, then the rate of interest is a local property of the function. In classical theory a local rate of interest is defined with application of classic calculus. The classic calculus is constructed with application of linear function. Classic derivative is the straight tangent line of the process. In financial mathematics is made the research of relationship between relative expansion of the value of the function and absolute expansion of the argument of the function. The adequate instrument of the research is not the straight tangent line but the exponential tangent line. With collection of exponential lines the construction of basic concepts of classic calculus is possible. The rate of interest is the exponential derivative in a point of the function. In financial mathematics this is adequate method of research.