ĐKXĐ :

`sin x - cos x \ne 0`

`<=> sin x \ne cos x`

`<=> sin x \ne sin(\pi/2 -x)`

`<=>` \(\left[ \begin{array}{l}x \ne \dfrac{\pi}{2} -x+k2\pi\\x \ne \pi - \dfrac{\pi}{2} +x+k2\pi\end{array} \right.\)    `(k \in ZZ)`

  `<=>` \(\left[ \begin{array}{l}x \ne \dfrac{\pi}{4} +k\pi\\0x \ne \dfrac{\pi}{2} +k2\pi  \ \rm ( \ vô \ lí \ )\end{array} \right.\)        `(k \in ZZ)`

`<=> x \ne \pi/4+k\pi`      `(k \in ZZ)`

Vậy  `D=R \\ {\frac{\pi}{4}+k\pi ; k \in ZZ}`

ĐKXĐ :

`sin x - cos x \ne 0`

`<=> sin x \ne cos x`

`<=> sin x \ne sin(\pi/2 -x)`

`<=>` \(\left[ \begin{array}{l}x \ne \dfrac{\pi}{2} -x+k2\pi\\x \ne \pi - \dfrac{\pi}{2} +x+k2\pi\end{array} \right.\)    `(k \in ZZ)`

  `<=>` \(\left[ \begin{array}{l}x \ne \dfrac{\pi}{4} +k\pi\\0x \ne \dfrac{\pi}{2} +k2\pi  \ \rm ( \ vô \ lí \ )\end{array} \right.\)        `(k \in ZZ)`

`<=> x \ne \pi/4+k\pi`      `(k \in ZZ)`

Vậy  `D=R \\ {\frac{\pi}{4}+k\pi ; k \in ZZ}`