# 查看完整版本 : 根式表示cos165度 的值??

kk5032019 2021-4-16 09:57

rightbright 2021-4-17 18:43

[quote]原帖由 [i]kk5032019[/i] 於 2021-4-16 09:57 AM 發表 [url=https://www.discuss.com.hk/redirect.php?goto=findpost&pid=534198121&ptid=29829722][img]https://www.discuss.com.hk/images/common/back.gif[/img][/url]

top11 2021-4-17 18:49

[quote]原帖由 [i]kk5032019[/i] 於 2021-4-16 09:57 AM 發表 [url=https://www.discuss.com.hk/redirect.php?goto=findpost&pid=534198121&ptid=29829722][img]https://www.discuss.com.hk/images/common/back.gif[/img][/url]

cos(A + B) = cos(A)cos(B) - sin(A)sin(B)

cos(165°)
= cos(120° + 45°)
= cos(120°)cos(45°) - sin(120°)sin(45°)
= cos(180° - 60°)cos(45°) - sin(180° - 60°)sin(45°)
= -cos(60°)cos(45°) - sin(60°)sin(45°)
= -(1/2)(√2)/2 - (√3)/2 × (√2)/2
= -(√2)/4 - √(6)/4
= [-(√2) - √(6)]/4

top11 2021-4-17 18:50

[quote]原帖由 [i]rightbright[/i] 於 2021-4-17 06:43 PM 發表 [url=https://www.discuss.com.hk/redirect.php?goto=findpost&pid=534260326&ptid=29829722][img]https://www.discuss.com.hk/images/common/back.gif[/img][/url]