pt: cos3x( 2cos2x + 1) = 1 /2
<=>2cos3x( 2cos2x + 1) = 1
<=> 4cos3xcos2x + 2cos3x = 1
<=> 2[cos(3x + 2x) + cos(3x - 2x)] + 2cos3x = 1
<=> 2cos5x + 2cosx + 2cos3x = 1
<=> 2cosx + 2cos3x + 2cos5x = 1
<=> 2cosxsinx + 2cos3xsinx + 2cos5xsinx = sinx
<=> sin2x + (sin4x - sin2x) + (sin6x - sin4x) = sinx
<=> sin6x = sinx
<=> \(\left[ \begin{array}{l}6x=x+k2 \pi \\6x= \pi-x+k2 \pi\end{array} \right.\)
<=>\(\left[ \begin{array}{l}x=\frac{k2\pi}{5} \\x=\frac{\pi}{7}+ \frac{k2\pi}{7} \end{array} \right.\)
Thay k=0 vào \(\left[ \begin{array}{l}x=\frac{k2\pi}{5} \\x=\frac{\pi}{7}+ \frac{k2\pi}{7} \end{array} \right.\) ⇒ nghiệm dương nhỏ nhất của pt là x = $\frac{\pi}{7}$
