Question Number 47641 by azharkhan250963@gmail.com last updated on 12/Nov/18
$$\mathrm{E}=\left(\mathrm{E}_{\mathrm{1}} ^{\mathrm{2}} +\mathrm{E}_{\mathrm{2}} ^{\mathrm{2}} +\mathrm{2E}_{\mathrm{1}} \mathrm{E}_{\mathrm{2}} \mathrm{cos2}\theta\right)^{\mathrm{1}/\mathrm{2}} \\ $$$$\mathrm{full}\:\mathrm{explanation} \\ $$