TRIGONOMETRI
2 sin 18° sin 306°
= 2 sin 18° . (-sin 54°)
= - cos (54 - 18)° + cos (54 + 18)°
= - cos 36° + cos 72°
= - 1/4 (1 + √5) + 1/4 (√5 - 1)
= -1/4 - 1/4
= -1/2 ✔
###
cos 36° = 1/4 (1 + √5)
cos 72° = 1/4 (√5 - 1)
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TRIGONOMETRI
2 sin 18° sin 306°
= 2 sin 18° . (-sin 54°)
= - cos (54 - 18)° + cos (54 + 18)°
= - cos 36° + cos 72°
= - 1/4 (1 + √5) + 1/4 (√5 - 1)
= -1/4 - 1/4
= -1/2 ✔
###
cos 36° = 1/4 (1 + √5)
cos 72° = 1/4 (√5 - 1)