Cara Menghitung Cos 18 derajat

Misal x = 18
cos 5x = cos 5.18
cos 5x = cos 90
cos 5x = 0
cos (x + 4x) = 0
cos x.cos 4x – sin x.sin 4x = 0
cos x (2cos2 2x – 1) – 2sin x sin 2x cos 2x = 0
cos x (2(2cos2 x – 1) – 1) – 4 sin2 x.cos x (2cos2 x – 1) = 0
4cos3 x – 3 cos x – 8 sin2 x.cos3 x + 4 sin2 x.cos x = 0
4cos3 x(1 – 2 sin2 x) + cos x (4sin2 x – 3) = 0
4cos3 x(2 cos2 x – 1) + cos x (4sin2 x – 3) = 0
8cos5 x – 4 cos3 x + 4cos x.sin2 x – 3cos x = 0 (cos x keluar)
cosx (8cos4 x – 4cos2 x + 4sin2 x – 3) = 0

(i)cos x = 0 ( TM )
(ii) 8cos4 x – 4cos2 x + 4sin2 x – 3 = 0
8cos4 x – 4cos2 x +2(2sin2 x – 1) – 1 = 0
8cos4 x – 4cos2 x +2(–cos 2x) – 1 = 0
8cos4 x – 4cos2 x +2(–2cos2 x + 1) – 1 = 0
8cos4 x – 4 cos2 x – 4cos2 x + 1 = 0
8cos4 x – 8cos2 x + 1 = 0
Gunakan rumus ABC untuk mencari akar2 persamaan
cos2 x =
=
cos x =
klo dihitung pake kalkulator sekitar 0.9

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