[填空题]
The following diagram shows a cell towqju,d7j v9 /doe n,*v3at3st t*t)wohdr $ \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 $\mathrm{C} is 45^{\circ}$ .
