given cos26=k find cos13

$$ cos(2θ) = 2 \times cos^2(θ) - 1 $$ $$cos^2(θ) = \frac{(cos(2θ) +1)}{2} $$ $$cos^2(13) = \frac {cos(26) +1}{2}$$ $$cos^2(13) = \frac{k +1}{2}$$ $$ \cos\:13\:=\pm\sqrt{\left(\frac{k+1}{2}\right)}} $$