Explanation
We have,
$$\cos 9\theta=\sin \dfrac{\pi}{4}$$
$$ \cos 9\theta =\cos \left( {{90}^{o}}-45^0 \right) $$
$$ 9\theta =45^0 $$
$$ 10\theta ={45}^{o} $$
$$ \theta =5^0$$
Value of $$\tan 6\theta $$
$$ =\tan 6\theta $$
$$ =\tan 6\times {{5}^{o}} $$
$$ =\tan {{30}^{o}} $$
$$=\dfrac{1}{\sqrt 3}$$
Hence, this is the answer.
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