`1) tan 2x ne 0`
`<=> 2x ne π/2 + kπ`
`<=> x ne π/4 + k(π)/2`
`=> D = RR` \ `{π/4 + k(π)/2}` `(k ∈ ZZ)`
`2) cos x ne 0`
`<=> x ne π/2 + kπ`
`=> D = RR` \ `{π/2 + kπ}` `(k ∈ ZZ)`
`3) x ne 2`
`<=> D = RR` \ `{2}`
`4) sin 4x ne 0`
`<=> 4x ne kπ`
`<=> x ne k(π)/4`
`=> D = RR` \ `{k(π)/4}` `(k ∈ ZZ)`
`5) 1 + cos x ne 0`
`<=> cos x ne -1`
`<=> x ne π + k2π`
`<=> D = RR` \ `{π + k2π}` `(k ∈ ZZ)`
