cosα-cos3α/sin3α-sinα请帮忙解答下.谢了.
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用a cos3a=cos(2a+a)
=cos2acosa-sin2asina
=(2cos²a-1)cosa-2sinacosa*sina
=2cos³a-cosa-2(1-cos²a)cosa
=4cos³a-3cosa
所以分子=4cosa-4cos³a
=4cosa(1-cos²a)
=4cosasin²a
同理sin3a=3sina-4sin³a
分母=2sina-4sin³a
=2sina(1-2sin²a)
原式=2sinacosa/(1-2sin²a)
=sin2a/coa2a
=tan2a