cosθ+cos(120°-θ)+cos(120°+θ)怎么化简,急用,

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cosθ+cos(120°-θ)+cos(120°+θ)怎么化简,急用,cosθ+cos(120°-θ)+cos(120°+θ)怎么化简,急用,cosθ+cos(120°-θ)+cos(120°+θ)

cosθ+cos(120°-θ)+cos(120°+θ)怎么化简,急用,
cosθ+cos(120°-θ)+cos(120°+θ)怎么化简,急用,

cosθ+cos(120°-θ)+cos(120°+θ)怎么化简,急用,
cosθ+cos(120°-θ)+cos(120°+θ)
=cosθ+cos120°cosθ+sin120°sinθ+cos120°cosθ-sin120°sinθ
=cosθ+2cos120°cosθ
=cosθ+2x(-1/2)cosθ
=0