cos$\frac{4x}{3}$ -$\frac{1+cos2x}{2}$ =0
đắt $\frac{2x}{3}$=a
-> cos2a- $\frac{1+cos3a}{2}$ =0
2cos2a-1-cos3a=0
2($2cos^{2}$a -1)-1-4$cos^{3}$a+3cosa=0
-4$cos^{3}$a+4$cos^{2}$a+3cosa-3=0
4$cos^{2}$a(1-cosa)-3(1-cosa)=0
(1-cosa)( 4$cos^{2}$a-3)=0
cosa=1 hoặc cosa=$\frac{\sqrt[]{3}}{2}$ hoặc cosa=-$\frac{\sqrt[]{3}}{2}$
a=k2$\pi$ hoặc a= ±$\frac{\pi}{6}$ + k2$\pi$ hoặc a= ±$\frac{5\pi}{6}$+k2 $\pi$
x=k3$\pi$ hoặc x=±$\frac{\pi}{4}$ + k3$\pi$ hoặc x=±$\frac{5\pi}{4}$+k3 $\pi$
