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công thức tổng -->tích:
cos2a +cos2b = 2cos(a+b).cos(a-b)
áp dụng:
<=> cos3x + (cosx +cos5x) = 0
<=> cos3x + 2cos3x.cos2x
<=> cos3x(1+ 2.cos2x) =0
=>[cos3x=0 => x=..
=>[cos2x =-1\2 => x=..
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công thức tổng -->tích:
cos2a +cos2b = 2cos(a+b).cos(a-b)
áp dụng:
<=> cos3x + (cosx +cos5x) = 0
<=> cos3x + 2cos3x.cos2x
<=> cos3x(1+ 2.cos2x) =0
=>[cos3x=0 => x=..
=>[cos2x =-1\2 => x=..