How to derive $P^2(,)=1-si{n}^{2}2co{s}^2,$ from $,E^2=E_Z^2+E_{Y^{\text{'}}}^2=\frac{e^4}{m^2{c}^4{R}^2}(E_{0Z}^2+E_{0Y}^{2}co{s}^2)$