$= \frac{{\sqrt 2 - \sqrt 2 \left\{ {\frac{1}{{\sqrt 2 }}\sin \alpha + \frac{1}{{\sqrt 2 }}\cos \alpha } \right\}}}{{\sqrt 2 \left\{ {\frac{1}{{\sqrt 2 }}\sin \alpha - \frac{1}{{\sqrt 2 }}\cos \alpha } \right\}}}$
$=\frac{{\sqrt 2 - \sqrt 2 \cos \left( {\alpha - \frac{\pi }{4}} \right)}}{{\sqrt 2 \sin \left( {\alpha - \frac{\pi }{4}} \right)}}$
$= \frac{{\sqrt 2 \left\{ {\,1 - \cos \theta } \right\}}}{{\sqrt 2 \sin \theta }},$ where $\theta = \alpha - \frac{\pi }{4}$
$= \frac{{2{{\sin }^2}(\theta /2)}}{{2\sin (\theta /2)\cos (\theta /2)}} = \tan \frac{\theta }{2}$
$ = \tan \left( {\frac{\alpha }{2} - \frac{\pi }{8}} \right)$.
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$(A)$ $\left(-\frac{1}{2},-\frac{1}{\sqrt{5}}\right)$$(B)$ $\left(-\frac{1}{\sqrt{5}}, 0\right)$
$(C)$ $\left(0, \frac{1}{\sqrt{5}}\right)$$(D)$ $\left(\frac{1}{\sqrt{5}}, \frac{1}{2}\right)$