a) <=> [TEX]2cos^2(x+\frac{\pi}{6}=2sin^2(3x)[/TEX]
<=>[TEX]2sin^2(\frac{\pi}{3}-x)=2sin^2(3x)[/TEX]
<=>[TEX]sin(\frac{\pi}{3})=sin3x [/TEX]
hoặc [TEX]-sin(\frac{\pi}{3}-x)=sin3x [/TEX]<=> [TEX]sin(x-\frac{\pi}{3})=sin3x[/TEX]
....
b)<=>[TEX]cos^24x=cos^2x[/TEX]
<=>cos4x=cosx
hoặc cos4x=-cosx <=> [TEX]cos4x=cos(\pi-x)[/TEX]