`15)`
`ĐKXĐ: x ne k2π`
`=> cos 2x - 1 = 0`
`<=> cos 2x = 1`
`<=> 2x = k2π`
`<=> x = kπ` `(k ∈ ZZ)`
`16)`
`ĐKXĐ:` \(\left\{ \begin{array}{l}x \ne \frac{π}{2} + kπ\\x \ne \frac{π}{4} + kπ\end{array} \right.\)
`=> x ne π/4 + kπ`
`=> cos 2x = 0`
`<=> 2x = ±(π)/2 + k2π`
`<=> x = ±(π)/4 + kπ`
`=> x = -(π)/4 + kπ, k ∈ ZZ`