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Diketahui :sin a - sin b = 2 cos (a+b)/2 sin (a-b)/2
cos a - cos b = -2 sin (a+b)/2 sin (a-b)/2
Dimana :
a = 40°
b = 20°
(sin 40° - sin 20°)/(cos 40° - cos 20°)
= 2 cos 30° sin 10° / -2 sin 30° sin 10°
= - cos 30° / sin 30°
= - √3