b) cos5x.sin4x = cos3x.sin2x
=>1/2sin9x - 1/2sinx = 1/2sin5x - 1/2sinx
=>1/2sin9x =1/2sin5x
=>sin9x = sin5x
=>9x = 5x + k.2pi hoặc 9x = pi-5x + k.2pi
=>4x = k.2pi hoặc 14x = pi + k.2pi
=>x = k.pi/2 hoặc x = pi/14 + k.pi/7
vậy: x = k.pi/2 hoặc x = pi/14 + k.pi/7

9) tan3x - tanx = -2sin2x ; dk: cos3x,cosx khác 0
<-> sin3x/cos3x - sinx/cosx = -2sin2x
<-> sin3x.cosx - cos3x.sinx = -2sin2x (bỏ mẫu luôn nha)
<-> sin2x = -2sin2x
<-> sin2x =0
<=> x = kπ

b) cos5x.sin4x = cos3x.sin2x
=>1/2sin9x - 1/2sinx = 1/2sin5x - 1/2sinx
=>1/2sin9x = 1/2sin5x
=> sin9x = sin5x
=> 9x = 5x + k.2pi hoặc 9x = pi - 5x + k.2pi
=> 4x = k.2pi hoặc 14x = pi + k.2pi
=>x = k.pi/2 hoặc x = pi/14 + k.pi/7
vậy x = k.pi/2 hoặc x = pi/14 + k.pi/7