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