The half angle identity to evaluate Cosπ/8 is Cosθ= 2Cos2 θ/2  -1

 

Substituting θ =π/4,   Cosπ/4  =  2Cos2 π/8 – 1

 

                          Thus Cos2 π/8 =

 

                                                  =

On simplification Cos2π/8 = = =.8546

 

                                Cosπ/8 = 0.9244

 

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