If $r = [2\phi + \cos^2(2\phi + \pi/4)]^{1/2}$,then what is the value of the derivative $dr/d\phi$ at $\phi = \pi/4$?

  • A
    $2\left( \frac{1}{\pi + 1} \right)^{1/2}$
  • B
    $2\left( \frac{2}{\pi + 1} \right)^{-1/2}$
  • C
    $2\left( \frac{1}{\pi + 1} \right)^{-1/2}$
  • D
    $2\left( \frac{2}{\pi + 1} \right)^{1/2}$

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