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数学
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已知sin(x-π/4)cos(x-π/4)=-1/4,求cos4x的值.
人气:111 ℃ 时间:2020-03-29 08:06:58
解答
sin(x-π/4)cos(x-π/4)
=1/2sin[2(x-π/4)]
=1/2sin(2x-π/2)
=-1/2sin(π/2-2x)
=-1/2cos2x=-1/4
cos2x=1/2
cos4x=2(cos2x)^2-1=-1/2
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