Question 769452
cosine is an even function so

{{{cos(-pi/8)=cos(pi/8)}}}


{{{cos((pi/4)/2)=sqrt((1+cos(pi/4))/2)}}}


{{{
sqrt((1+cos(pi/4))/2)=sqrt((1+1/sqrt(2))/2)=sqrt(2+sqrt(2))/2}}}