v 10.2

ε_{0} χ_{2} E_{0}^{2} \cos^{2} ω t = ε_{0} χ_{2} E_{0}^{2} [\frac{1}{2} (1 + \cos 2 ω t)]

${\varepsilon_0 \chi_2 E_0^2 \cos^2 \omega t=\varepsilon_0 \chi_2 E_0^2 \bigg[\frac{1}{2} (1+ \cos 2\omega t)\bigg]}$