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求值:cos20-2sin20sin40
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求值:cos20-2sin20sin40
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题目漏掉了“°”吧?
cos20°-2sin20°sin40°
=cos20°-4(sin20°)^2cos20°
=cos20°-4[1-(cos20°)^2]cos20°
=4(cos20°)^3-3cos20°
=cos(3×20°)
=cos60°
=1/2.
cos20°-2sin20°sin40°
=cos20°-4(sin20°)^2cos20°
=cos20°-4[1-(cos20°)^2]cos20°
=4(cos20°)^3-3cos20°
=cos(3×20°)
=cos60°
=1/2.
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