2(sin4 п/8-cos4 п/8)

[latex]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} [/latex]