作业帮cos(5*3.1415926..)/8 * cos (3.1415926../8)=多少3.1415926＝派
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cos(5*3.1415926..)/8 * cos (3.1415926../8)=多少3.1415926＝派
cos(5*3.1415926..)/8 * cos (3.1415926../8)=多少
3.1415926＝派
cos(5*3.1415926..)/8 * cos (3.1415926../8)=多少3.1415926＝派
cos（5π/8） × cos（π/8）
=[cos（π/2+π/8）]×cos （π/8）
=-sin（π/8）×cos （π/8）----公式：cosπ/2+a=-sina
=-1/2 × 2sin（π/8）×cos （π/8）
-----后面的2sincos用2倍角公式合并
=-1/2 × sin （π/4）
=-1/2 × √2/2
=-√2/4
(cos(5 * 3.1415926) / 8) * cos(3.1415926 / 8) = -0.115484942
cos(5*3.1415926..)/8 * cos (3.1415926../8)=多少3.1415926＝派
cos（5π/8）*cosπ/8
cos(5π/8)*cos（π/8)=
化简cos^8(π/8)+cos^8(3π/8)+cos^8(5π/8)+cos^8(7π/8)
cos^2 π/8+cos ^2 3π/8+ cos^2 5π/8+cos^2 7π/8=?
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cos 5π/8 cos π/8 + sin 5π/8 sin π/8=
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cos=-3/5,
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