Show that the spacetime interval given by Eq. 15.18 is invariant with respect to the Lorentz transformation. That is, show that $(d s)^{2}=\left(d s^{\prime}\right)^{2},$ where $\left(d s^{\prime}\right)^{2}=$ $\left(c d r^{\prime}\right)^{2}-\left(d x^{\prime}\right)^{2}$