cos(x/2) when tan x is given and x lies in the third quadrant

1. A.
   $\frac{5}{\sqrt{13}}$
2. B.
   $\frac{5}{\sqrt{26}}$
3. C.
   $\frac{5}{13}$
4. D.
   $\sqrt{\frac{1}{26}}$

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1. A.
   $-\frac{2}{\sqrt{5}}$
2. B.
   $\frac{2}{\sqrt{5}}$
3. C.
   $\frac{1}{\sqrt{10}}$
4. D.
   $-\frac{1}{\sqrt{10}}$

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