# Rotational symmetry

 P: 4 I haven't watched the video, but if you perform a rotation by an angle $\theta$ about the $z$ axis on the vector ${\bf r} = \left(x,y,z\right)$, you get $r' = r + \Delta r = \left(\cos \theta x - \sin \theta y, \sin \theta x + \cos \theta y ,z\right)$. For $\theta = \epsilon$ infinitesimal, this becomes $r' = r + \delta r = \left(x - \epsilon y, \epsilon x + y ,z\right) = r + \left(- \epsilon y, \epsilon x ,0\right)$, so that $\delta x = - \epsilon y$ and $\delta y = - \epsilon x$.