\(\overline{DE}=4\),\(\displaystyle\overline{AD}=4+2\left(4\cos\frac{9\pi}{14}\right)\),\(\displaystyle ∠ ADE=\frac{\pi}{14}\)
所求=\(\displaystyle-\overline{DE}\cdot\overline{AD}\cos \frac{\pi}{14}=16\left(2\sin\frac{\pi}{7}-1\right)\cos\frac{\pi}{14}.\)
不知道要(能)化簡到怎樣的地步?