`sin x - cos x \ne 0`
`<=> sin x \ne cos x`
`<=> sin x \ne sin (π/2 - x)`
`<=>` \(\left[ \begin{array}{l}x = \dfrac{π}{2} - x + k2π\\x = \dfrac{π}{2} + x + k2π\end{array} \right.\)
`=> 2x = π/2 + k2π`
`<=> x = π/4 + kπ`
`=> D = RR \\ {π/4 + kπ | k ∈ ZZ}`