Bài 2:
a, ( x + 1 ) ( 3 - x ) = 0
`=>` \(\left[ \begin{array}{l}x+1=0\\3-x=0\end{array} \right.\)
`=>` \(\left[ \begin{array}{l}x=-1\\x=3\end{array} \right.\)
Vậy x ∈ { -1 ; 3 }
b, ( x - 2 ) ( 2x - 1 ) = 0
`=>` \(\left[ \begin{array}{l}x-2=0\\2x-1=0\end{array} \right.\)
`=>` \(\left[ \begin{array}{l}x=2\\x =\frac{1}{2}\end{array} \right.\)
Vậy x ∈ { 2 ; `1/2` }
c, ( 3x + 9 ) ( 1 - 3x ) = 0
`=>` \(\left[ \begin{array}{l}3x+9=0\\1-3x=0\end{array} \right.\)
`=>` \(\left[ \begin{array}{l}x=-3\\x=\frac{1}{3}\end{array} \right.\)
d, ( x² + 1 ) ( 81 - x² ) = 0
`=>` \(\left[ \begin{array}{l}x^2+1=0\\81-x^2=0\end{array} \right.\)
`=>` \(\left[ \begin{array}{l}x=-1(vô lí)\\x=9\end{array} \right.\)
`=>` \(\left[ \begin{array}{l}x^2=-1(loại)\\x=±9\end{array} \right.\)
Vậy x ∈ { ±9 }
e, `(x-5)^5` = 32
`(x-5)^5` = `2^5`
`=>` x - 5 = 5
`=>` x = 10
Vậy x = 10
f, `(2-x)^4` = 81
`=>` `(2-x)^4` = `3^4`
`=>` 2 - x = 3
`=>` x = -1
Vậy x = -1
g, `(31-2x)^3` = -27
`(31-2x)^3` = `(-3)^3`
`=>` 31 - 2x = -3
`=>` 2x = 34
`=>` x = 17
Vậy x = 17
h, ( x - 2 ) ( 7 - x ) > 0
`=>` x - 2 và 7 - x cùng dấu
+, Với x - 2 > 0 `=>` 7 - x > 0
`=>` x > 2 và x < 7 ( t/m )
+, Với x - 2 < 0 `=>` 7 - x < 0
`=>` x < 2 và x > 7 ( vô lí )
Vậy 2 < x < 7
i, | x - 7 | ≤ 3
`=>` x - 7 ∈ { 0 ; ±1; ±2 ; ±3 }
`=>` x ∈ { 7; 8; 6; 9; 5; 10; 4 }
Vậy x ∈ { 7; 8; 6; 9; 5; 10; 4 }
