Energy principle and variational method note 14: summary + problem solving

$$\begin{gathered} The\; principle\; of\; minimum\; potential\; energy\; is\; the\; true\; solution \\ The\; first\; two\; cases\; are\; possible\; states\; ,\; We\; need\; to\; add\; the\; constitutive\; equation\; ,\; Geometric\; equations\; ,\; Displacement\; boundary \end{gathered}$$

Situation 1

Situation two

The principle of virtual displacement and differential method can be deduced from energy

solve ？

$$If\; the\; exact\; solution\; is\; obtained,\; it\; is\; recommended\; to\; use\; the\; infinitesimal\; method\; ,\; If\; the\; approximate\; solution\; is\; found,\; the\; method\; of\; work\; and\; energy\; is\; recommended$$