F.3 gradient.

如果我冇計錯....答案應該係A...

個步驟如下:

In triangle ABD,
tanθ= h / BD
BD = h / tanθ ...... 1

In triangle BCD
tan x = CD / BD (唔識打個角度...用 x 代住先)
BD = CD/ tan x ....... 2

sub. 1 into 2,
CD / tan x = h / tanθ
CD = h / tanθ * tan x
= h tan x / tan θ

所以個答案係A.....應該冇計錯= =...
依個係我第一次上黎答野....希望幫到你!!