Đáp án:
\(\left[ \begin{array}{l}x=-7\pi/12 -k2\pi \\x=-13\pi/36 +k2\pi/3\end{array} \right.\)
Giải thích các bước giải:
$Cos( x +\pi/4) = -cos(\pi/6 -2x)$
$<=> cos( x +\pi/4) = cos(5\pi/6 +2x)$
<=>\(\left[ \begin{array}{l}x+\pi/4=5\pi/6 +2x +k2\pi\\x+\pi/4= -5\pi/6 -2x +k2\pi
\end{array} \right.\)
<=>\(\left[ \begin{array}{l}x=-7\pi/12 -k2\pi\\3x=-13\pi/12 +k2\pi\end{array} \right.\)
<=> \(\left[ \begin{array}{l}x= -7\pi/12 -k2\pi\\x=-13\pi/36 +k2\pi/3\end{array} \right.\)
