Dik ketahui sin x =8/10, maka nilai cos 3x+cosx=

Sin x = 4/5
Sin = De/Mi
Sa = √5²-4² = √9 = 3
Cos x = 3/5
Cos (2x+x) = cos(2x).cos(x)-sin(2x).sin(x)
= cos²(x)-sin²(x).3/5-2sin(x).cos(x).4/5
= 9/25-16/25.3/5-2.4/5.3/5.4/5
= -7/25.3/5 - 2(48/125)
= -21/125 - 96/125 = -117/125
cos3x + cosx
=-117/125 + 3/5
=-117+75/125 = -42/125

Trigonometri

sin x = 8/10 = 4/5
cos x = √(1 - sin² x) = √(1 - (4/5)²) = √(9/25)
cos x = 3/5

cos 3x  + cos x =
= cos (2x + x) + cos x
= cos 2x cos x - sin 2x sin x + cos x
= cos x ( 2 cos² x  -1) - (2 sin x cos x) sin x + cos x
= (3/5) {2(3/5)² -1} - { 2(4/5)(3/5)(4/5)} + 3/5
= (3/5)( -7/25) - (96/125) + 3/5
= - 21/125 - 96/125 + 3/5
= - 21/125  - 96/125 + 75/125
= -42/125