This document discusses shape functions in finite element analysis. It explains that shape functions approximate the variation of displacement within an element since the true variation is unknown. Shape functions, also called interpolation functions, are used to replace difficult analytical solutions with easier mathematical functions. Important properties of shape functions are that the magnitude at each node is unity, the number of functions equals the number of nodes, and the sum of functions is always unity. Common methods for deriving shape functions include using polynomials in Cartesian and natural coordinate systems and Lagrange interpolation in natural coordinates.