こんな感じで加法定理で求められます
cos75°
= cos(45°+30°)
= cos45°cos30° - sin45°sin30°
= (1/√2)×{ (√3)/2 } - (1/√2)×{1/2}
= { (√3)/(2√2) } - { 1/(2√2) }
= { (√3) - 1 }/(2√2)
= { (√6) - (√2) }/4
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