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

Question

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

Accepted Answer

$2\,{\left( {\frac{2}{{\pi  + 1}}} \right)^{1/2}}$

Answer

$2\,{\left( {\frac{1}{{\pi  + 1}}} \right)^{1/2}}$

Answer

$2\,{\left( {\frac{2}{{\pi  + 1}}} \right)^{ - 1/2}}$

Answer

$2\,{\left( {\frac{1}{{\pi  + 1}}} \right)^{ - 1/2}}$

differentiation