cos2x cosπ/5+sin2x sinπ/5化简最详细的步骤?

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cos2x cosπ/5+sin2x sinπ/5化简最详细的步骤?

两角差的余弦=cos(2x -π/5)

cos2x cosπ/5+sin2x sinπ/5
=cos(2x-π/5)