`text{Câu 1}`
`ĐKXĐ:`
\(\left\{ \begin{array}{l}21 - x ≥ 0\\21 + x ≥ 0\\2x^2 + 18 \ne 0\\-x^2 + 81 \ne 0\\7 - x > 0\end{array} \right.\)
`->` \(\left\{ \begin{array}{l}x ≤ 21\\x ≥ -21\\x^2 \ne -9\ (luôn\ đúng)\\x \ne ±9\\x < 7\end{array} \right.\)
`->` \(\left\{ \begin{array}{l}-21 ≤ x < 7\\x \ne -9\end{array} \right.\)
`-> D = [-21; 7) \\ {-9}`
`text{Câu 2}`
`D = [-5/4; +infty)`
`sqrt{4x + 5} - sqrt{3x^2 + 4x + 2} = 0`
`->sqrt{4x + 5} = sqrt{3x^2 + 4x + 2}`
`-> 4x + 5 = 3x^2 + 4x + 2`
`-> 3x^2 = 3`
`-> x^2 = 1`
`-> x = +-1` `(TM)`
`-> S = {+-1}`
`text{Câu 3}`
`D = RR`
`|5x^2 + 10x + 17| = |2x^2 - 7x + 7|`
`->` \(\left[ \begin{array}{l}5x^2 + 10x + 17 = 2x^2 - 7x + 7\\5x^2 + 10x + 17 = -2x^2 + 7x - 7\end{array} \right.\)
`->` \(\left[ \begin{array}{l}3x^2 + 17x + 10 = 0\\7x^2 + 3x + 24 = 0\ (vô\ nghiệm)\end{array} \right.\)
`->` \(\left[ \begin{array}{l}x = \dfrac{-2}{3}\\x = -5\end{array} \right.\)
`-> S = {-5; -(2)/(3)}`
