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
*****