This manuscript, titled Differential Geometry of Curves and Surfaces in Three-dimensional Euclidean Space, is intended for undergraduate students of mathematics and other sciences with the need of use of Euclidean differential geometry. We deal with the differential geometry of curves and surfaces in three dimensions, covering all the main definitions, ideas and concepts needed for successful understanding and further application of this fundamental branch of mathematics. Many concrete examples with solutions are given with illustrations. ----- Ova knjiga namijenjena je dodiplomskim studentima matematike i drugih nauka sa potrebom upotrebe euklidske diferencijalne geometrije. Upoznat \'cemo se sa diferencijalnom geometrijom krivih i povr\v{s}i u tri dimenzije, pokrivaju\'ci sve glavne definicije, ideje i koncepte potrebne za razumijevanje i uspje\v{s}nu dalju primjenu ove fundamentalne grane matematike. Dati su mnogi konkretni problemi sa rje\v{s}enjima i mno\v{s}tvom ilustracija.

This manuscript, titled Differential Geometry of Curves and Surfaces in Three-dimensional Euclidean Space, is intended for undergraduate students of mathematics and other sciences with the need of use of Euclidean differential geometry. We deal with the differential geometry of curves and surfaces in three dimensions, covering all the main definitions, ideas and concepts needed for successful understanding and further application of this fundamental branch of mathematics. Many concrete examples with solutions are given with illustrations. ----- Ova knjiga namijenjena je dodiplomskim studentima matematike i drugih nauka sa potrebom upotrebe euklidske diferencijalne geometrije. Upoznat \'cemo se sa diferencijalnom geometrijom krivih i povr\v{s}i u tri dimenzije, pokrivaju\'ci sve glavne definicije, ideje i koncepte potrebne za razumijevanje i uspje\v{s}nu dalju primjenu ove fundamentalne grane matematike. Dati su mnogi konkretni problemi sa rje\v{s}enjima i mno\v{s}tvom ilustracija.

The aim of this paper is to explore in some detail the second order linear ordinary differential equation with real or complex periodic coefficients, also known as the Hill’s equation, with some emphasis on stability and instability intervals and explore two related self-adjoint eigenvalue problems leading to the two final results which enable us to practically solve problems of this type.