Show that if $($x$1,$ y$1,$ z$1,$ t$2)$ and $($x$2,$ y$2,$ z$2,$ t$2)$ are the coordinates of an event in S and the corresponding event in S$2,$ respectively, then the expression ds$,^2=$ dx$^2+$ dy$^2+$ dz$,^2-$ c$^2$dt$,^2$ is variant under a Lorentz coordinate transformation $($i$.$e$.$ ds$,2=$ ds$,2).$