cos3x/sinx+sin3x/cosx (通分)
=(cos3xcosx+sin3xsinx)/(sinxcosx) (分子用积化和差,分子用倍角公式)
=cos(3x-x)/(1/2sin2x)
=2cos2x/sin2x
=2cot2x.
证明cos3x/sinx+sin3x/cosx=2cot2x
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