testing test test S [ x ( τ ) + δ x ( τ ) ] − S [ x ( τ ) ] = 0 {\displaystyle \ S[x(\tau )+\delta x(\tau )]-S[x(\tau )]=0}
![{\displaystyle \ S[x(\tau )+\delta x(\tau )]-S[x(\tau )]=0}](http://en.wikipedia.beta.wmflabs.org/api/rest_v1/media/math/render/svg/a378e54f6804311c88afd6dfa2b6ef8287c3d7d8)