If $x + y = 3 - cos4\theta$ and $x - y = 4 \,sin2\theta$ then

  • A

    $x^4 + y^4 = 9$

  • B

    $\sqrt x \, + \,\sqrt y \, = \,16\,$

  • C

    $x^3 + y^3 = 2(x^2 + y^2)$

  • D

    $\sqrt x \, + \,\sqrt y \, = \,2$

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Suppose $\theta $ and $\phi  (\ne 0)$ are such that $sec\,(\theta  + \phi ),$ $sec\,\theta $ and $sec\,(\theta  - \phi )$ are in $A.P.$ If $cos\,\theta  = k\,cos\,( \frac {\phi }{2})$ for some $k,$ then $k$ is equal to

  • [AIEEE 2012]

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