Let

1. Formulation of the problem In practical problem with numeric description of heat flowing [1] (Fourier equa-tion), the diffusion (Fick equation), etc. there are large linear system equations with 3-band matrices. In the paper we give the direct models for the determinants (proper-ty 1 and 4) and the algebraic complement of that matrices (property 2 and 5). In the result we receive the direct models for solving the linear systems equations expressed by the 3-band matrices (property 3 and 6). 2. Solution of the proble