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cos75cos15-cos165sin165
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cos75cos15-cos165sin165
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答案和解析
cos75°cos15°-cos165°sin165°
=cos75°cos15°-cos(90°+75°)sin(180°-15°)
=cos75°cos15°-(-sin75°)sin15°
=cos75°cos15°+sin75°sin15°
=cos(75°-15°)
=cos60°
=2分之1
=cos75°cos15°-cos(90°+75°)sin(180°-15°)
=cos75°cos15°-(-sin75°)sin15°
=cos75°cos15°+sin75°sin15°
=cos(75°-15°)
=cos60°
=2分之1
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