Kak delat etu ☝ pomagitee

[latex] cos^{2} \alpha - sin^{2} \alpha =cos(2* \alpha )[/latex] [latex] cos^{2} \frac{ \pi }{8}- sin^{2} \frac{ \pi }{8}=cos(2* \frac{ \pi }{8})=cos \frac{ \pi }{4}= \frac{ \sqrt{2} }{2} [/latex]