cos4 pi /8+ cos4 3pi /8+ cos4 5pi /8 +cos4 7pi/ 8=3/2  prove it fast rely plssss

 Cos&^4 pi/8+cos^4 3pi/8+ [cos (pi-3pi/8)]^4+cos(pi-pi/8)^4]

= 2cos^4 pi/8 +2cos^4 3pi/8

=1/2[(1+cospi/4)^2+ (1+cos3pi/4)^2]

=1/2[(1+1/root2)^2 + (1-1/root2)^2]

=1/2(2+1)

 =3/2 proved!