Cos 150 derajat senilai dengan – Cos 30 = 
.
Pembahasan
Perbandingan sudut dalam trigonometri.
Dalam trigonometri, besar sudut α dibagi menjadi 4 kelompok, yaitu :
Kuadran I : 0° < α < 90° (0 < α < π/2)
Sin (90° – α) = cos α
Cos (90° – α) = sin α
Tan (90° – α) = cotan α
Cotan (90° – α) = tan α
Secan (90° – α) = sec α
Cosec (90° – α) = cosec α
Kuadran II : 90° < α < 180° (π/2 < α < π)
Sin (180° – α) = sin α
Cos (180° – α) = -cos α
Tan (180° – α) = -tan α
Cotan (180° – α) = -cotan α
Secan (180° – α) = secan α
Cosec (180° – α) = -cosec α
Kuadran III : 180° < α < 270° (π < α < 3π/2)
Sin (270° – α) = -cos α
Cos (270° – α) = -sin α
Tan (270° – α) = cotan α
Cotan (270° – α) = tan α
Secan (270° – α) = -sec α
Cosec (270° – α) = -cosec α
Kuadran IV : 270° < α < 360° (3π/2 < α < 2π)
Sin (360° – α) = sin (-α) = -sin α
Cos (360° – α) = cos (-α) = cos α
Tan (360° – α) = tan (-α) = -tan α
Cotan (360° – α) = cotan (-α) = -cotan α
Secan (360° – α) = secan (-α) = -secan α
Cosec (360° – α)= sec (-α) = sec α
Penyelesaian Soal
Cos 150 berada di kuadran 2
Cos 150 = Cos (180 – 30)
= – cos 30
= .
