### Distance element

Two different coordinate systems are established on a plane, $(x,y)$ and $(x',y')$, which are related as follows:
$x'=x\cos\theta -y\sin\theta, \quad y'=x\sin\theta +y\cos\theta$
where $\theta=\mbox{const}$. Find the distance element in terms of coordinates $(x',y')$, if $(x,y)$ are Cartesian.