In the paper, theoretical knowledge about base solution of common differential equations with variable parameters is presented. The solutions are applied to mechanical problems of discrete, distributed and discrete-distributed homogeneous elastic models of structures and structural components. In this paper, the influence function and its properties are presented. The influence function is applied to analysis of the bending curve of a beam with constants and variable parameters. The presented method of the influence function is based on the mathematical similarity of differential equations describing free vibrations and deflection of beams, which are fourth-order equations with variable coefficients. In this paper; examples of calculation of support reactions as function of stiffness of the beam and elastic supports are presented.