Mohon bantuanyaaaaaaa

BC = CD = a => BD = 2a

tan x = AB/BC
AB = BC Tan x = a Tan x

AD = √(BD^2 + AB^2)
= √((2a)^2 + (a Tan x)^2)
= √(4a^2 + a^2 tan^2 x)
= √(a^2 (4 + tan^2 x))
= a √(4 + tan^2 x)

Cos D = BC/AD = (2a) / (a√(4 + tan^2 x)) = 2/√(4 + tan^2 x)