Tan=4/3 untuk 0900, berapa nilai cos 3x + cos x =?

[Trigonometri]

Batas 0° < x < 90° (Kuadran I)

Tan x = 4/3
Sin x = 4/5
Cos = 3/5

Maka...
Cos 3x + Cos x
= 2Cos 1/2(3x + x) Cos 1/2(3x - x)
= 2Cos 2x Cos x
= 2(1 - 2Sin²x)Cos x
= (2 - 4Sin²x)Cos x
= (2 - 4(4/5)²)(3/5)
= (2 - 4(16/25))(3/5)
= (2 - 64/25)(3/5)
= (-14/25)(3/5)
= -42/125