Ingat:
Cos a + cos b = 2 cos 1/2(a+b) cos 1/2 (a-b)
Cos 80 + cos 40 = 2 cos 1/2 (80 + 40) cos 1/2(80-40)
= 2 cos 1/2 (120) cos 1/2 (40)
= 2 cos 60 cos 20
= 2 . 1/2 cos 20
=Cos 20
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Ingat:
Cos a + cos b = 2 cos 1/2(a+b) cos 1/2 (a-b)
Cos 80 + cos 40 = 2 cos 1/2 (80 + 40) cos 1/2(80-40)
= 2 cos 1/2 (120) cos 1/2 (40)
= 2 cos 60 cos 20
= 2 . 1/2 cos 20
=Cos 20