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If cosbeta is the geometric mean betwee...

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)``

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