sin2x-cos2x=根号2·(sin2x·根号2/2-cox2x·根号2)
=根号2·(sin2x·cosπ/4-cos2x·sinπ/4)
=根号2·sin(2x-π/4)(逆用差角的正弦公式)
sin2x-cos2x=根号2·sin(2x-π/4)?怎么变的
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