1、18.5 m
2、C
3、设AB = x,则x(tan23° - tan20°) = 30,
∴ x ≈ 496(m)
4、设AB = x,则x(tan65°13′- tan45°) = 23.
∴ x ≈ 19.73,BC = 19.73 + 23 = 42.73(m).
5、BC = CD = 3.2 m,AC = BC,tan60° ≈ 5.54(m)> 4.5 m,担心有必要.
6、作CD ⊥ AB,垂足为D,
AB = 10,cos30° + 10sin30° ≈ 13.66,
AC + BC = 10 + 10,
sin30°÷sin45° ≈ 17.071 - 13.66 ≈ 3.4(m)