$\frac{d^2}{dx^2}(2\cos x \cos 3x) = $

  • A
    $2^2(\cos 2x + 2^2\cos 4x)$
  • B
    $2^2(\cos 2x - 2^2\cos 4x)$
  • C
    $2^2(-\cos 2x + 2^2\cos 4x)$
  • D
    $-2^2(\cos 2x + 2^2\cos 4x)$

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