三角恒等变形一题求助,第15题
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cos7π/8=-cosπ/8
cos5π/8=-cos3π/8=-sinπ/8
(cosπ/8)^4+(cos3π/8)^4+(cos5π/8)^4+(cos7π/8)^4
=(cosπ/8)^4+(sinπ/8)^4+(-cos3π/8)^4+(-cosπ/8)^4
=2[(sinπ/8)^4+(cosπ/8)^4]
=2[(cos²π/8+sin²π/8)²-2sin²π/8cos²π/8]
=2[1-(sin²π/4)/2]
=2(1-1/4)
=3/2
cos5π/8=-cos3π/8=-sinπ/8
(cosπ/8)^4+(cos3π/8)^4+(cos5π/8)^4+(cos7π/8)^4
=(cosπ/8)^4+(sinπ/8)^4+(-cos3π/8)^4+(-cosπ/8)^4
=2[(sinπ/8)^4+(cosπ/8)^4]
=2[(cos²π/8+sin²π/8)²-2sin²π/8cos²π/8]
=2[1-(sin²π/4)/2]
=2(1-1/4)
=3/2
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你的结果好像不对
追答
这是对的呀
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