If `cosbeta` is the geometric mean between `sinalphaa n dcosalpha,` where `0ltalpha,betaltpi/ 2,`then`cos2beta` is equal to(a)`-2sin^2(pi/ 4-alpha)` (b) `-2cos^2(pi/ 4+alpha)` (c)`-2sin^2(pi/4+alpha) (d)``2cos^2(pi/ 4-alpha)``