therefore
cos2(π/4-x)
= 1-2sin^2(π/4-x)
=7/25
=cos(π/2-2x)
=sin2x
Because x belongs to (σ/4, σ/2), so
2x belongs to (Wu /2, Wu)
cos2x & lt0
cos2x=-√ 1-sin^22x=-√ 1-(7/25)^2=-24/25