11)
sin^3(x) - √3cos^3(x)=sinx.cos^2(x)-√3sin^2(x)cosx
<=> sin^3(x) - sinxcos^2(x) = căn(3)cos^3(x) - căn(3)sin^2(x)cosx
<=> sinx [sin^2(x) - cso^2(x)] = căn(3)cosx [cos^2(x) - sin^2(x)]
<=> -sinx.cos2x = căn(3)cosx.cos2x
cũng có 2 TH như trên :
TH1 : cos2x = 0
<=> 2x = pi/2 + kpi <=> x = pi/4 + kpi/2
TH2 : cos2x khác 0
=> -sinx.cos2x = căn(3)cosx.cos2x
<=> -sinx = căn(3)cosx
<=> -tgx = căn(3)
<=> x = -pi/3 + k2pi

14)

2sin(x+pi/4)=1/sinx+1/cosx