This book presents a unified approach to classical approximation methods in engineering by applying the weighted residual method to transform differential equations into solvable algebraic systems. It demonstrates how this procedure underlies the finite difference, finite element, finite volume, and boundary element methods. The mechanical focus is on the one-dimensional tensile bar, allowing the mathematical framework and resulting matrix equations to be fully displayed and understood without symbolic abstraction. This approach supports a clear understanding of the derivation processes and is designed to help readers implement and extend features such as constitutive models in commercial simulation tools.