## Fermat’s Principle

Fermat’s Principle is

$$\delta s=\delta\int n\dd \ell=0$$

If we choose the path …

## Invariance of Euler-Lagrange Equations

E-L is deduced from the Hamilton’s principle

\delta S=\delta\int L(\bm q, \dot{\bm q}, t)dt …

## Energy of a triangle

For a triangle $$ABC$$, assume the points have potential $$\varphi_a,\varphi_b,\varphi_c$$. As shown in Fig. ???, \(AB'\perp AC, AC' …