2(sin4 п/8-cos4 п/8)

[tex]2*(sin ^{4} \frac{ \pi }{8} -cos ^{4} \frac{ \pi }{8} )= \\ =-2(cos ^{4} \frac{ \pi }{8}-sin ^{4} \frac{ \pi }{8} ) = \\ =-2(cos2* \frac{ \pi }{8} )=-2*cos \frac{ \pi }{4} = \\ =-2* \frac{ \sqrt{2} }{2} =- \sqrt{2} [/tex]