[填空题]
1. Show that $\log _{16}(\cos 2 x+7)=\log _{4} \sqrt{\cos 2 x+7}$ .$\log _{a} \sqrt{\cos b x+c}$ a = b = c =
2. Hence, or otherwise, solve $\log _{4}(\sqrt{10} \cos x)=\log _{16}(\cos 2 x+7)$ , for $0\lt x\lt \frac{\pi}{2}$ .$\frac{\pi}{a}$ a =
