b: \(\Leftrightarrow1-cos^2x-cosx+1=0\)
\(\Leftrightarrow cos^2x-cosx-2=0\)
\(\Leftrightarrow cosx=-1\)
hay \(x=\Pi+k2\Pi\)
d: \(\Leftrightarrow2cos^2x-1+9cosx+5=0\)
\(\Leftrightarrow2cos^2x+9cosx+4=0\)
\(\Leftrightarrow2cos^2x+8cosx+cosx+4=0\)
\(\Leftrightarrow cosx=-\dfrac{1}{2}\)
hay \(x=\pm\dfrac{2\Pi}{3}+k2\Pi\)