2cosx-1=0
=>cosx=1/2
=>x=pi/3+k2pi hoặc x=-pi/3+k2pi
Đối với x=-pi/3+k2pi thì chúng ta có thể lấy k=0; k=1; k=2;k=3;k=4;k=5
Đối với x=pi/3+k2pi thì chúng ta có thể lấy k=0;k=1;k=2;k=3;k=4
=>Có 11 nghiệm
\(cosx=\dfrac{1}{2}\Rightarrow\left[{}\begin{matrix}x=\dfrac{\pi}{3}+k2\pi\\x=-\dfrac{\pi}{3}+n2\pi\end{matrix}\right.\)
\(\left\{{}\begin{matrix}-\dfrac{3\pi}{2}\le\dfrac{\pi}{3}+k2\pi\le10\pi\\-\dfrac{3\pi}{2}\le-\dfrac{\pi}{3}+n2\pi\le10\pi\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}-\dfrac{11}{12}\le k\le\dfrac{29}{6}\\-\dfrac{7}{12}\le n\le\dfrac{31}{6}\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}k=\left\{0;1;2;...;4\right\}\\n=\left\{0;1;2;...;5\right\}\end{matrix}\right.\) có 5+6=11 nghiệm
