作者cool791010 (傑哥)
看板Math
標題Re: [中學] 三角函數一題
時間Wed Oct 10 01:31:55 2012
※ 引述《sincetoday (sincetoday)》之銘言:
: sin20sin40sin80 =? (單位為度)
: 今天碰到這題一時想不出來,
: 謝謝!
sinθsin(60-θ)sin(60+θ)
=sinθ(sin60cosθ-cos60sinθ)(sin60cosθ+cos60sinθ)
2 2 2 2
=sinθ[(sin 60)(cos θ)-(cos 60)(sin θ)]
2 2
=sinθ[ 3/4 *(1-sin θ)-1/4 *sin θ]
2
=sinθ[(3-4sin θ)/4]
3
=(3sinθ-4sin θ)/4]
=sin3θ/4
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推 sincetoday :謝謝! 10/10 01:48
推 citynew :可以請問除此與tanθtan(60+θ)tan(60-θ)=1/3tanθ 10/11 06:05
→ citynew :外,cos也有類似性質嗎?? 10/11 06:06