[填空题]
The following diagraa37aaw1 bb.t pkowp,ry a/ 8u(ohnwm ;1u.9 qqgh8z*b53wd oqm shows a cell tower $ \mathrm{AB} 6.4 \mathrm{~m} $ tall on the roof of a commercial building. The angle of depression from $\mathrm{A} to a point \mathrm{C} $ on the horizontal ground is $49^{\circ}$. The angle of elevation of the top of the building from $\mathrm{C} is 45^{\circ}$ .
