Bài 4:
12) x/3 - 2/3 = -4/7. 21/8
x/3 - 2/3 = -3/2
x/3 = -3/2 + 2/3
x/3 = -5/6
$\frac{x.2}{6}$ = -5/6
x.2 = -5
x = -5/2
Vậy x = -5/2
13) $\frac{x-1}{9}$ = 8/3
$\frac{x-1}{9}$ = $\frac{8.3}{9}$
x - 1 = 24
x = 24 + 1
x = 25
Vậy x = 25
14) $\frac{x-2}{5}$ = $\frac{x+5}{4}$
$\frac{4(x-2)}{5.4}$ = $\frac{5(x+5)}{4.5}$
$\frac{4x-8}{20}$ = $\frac{5x+25}{20}$
4x - 8 = 5x + 25
4x - 5x = 25 + 8
-x = 33
x = -33
Vậy x = -33
15) Thiếu
16) Thiếu
17) Thiếu
18) (2x - 3)(6 - 2x) = 0
2x - 3 = 0 hoặc 6 - 2x = 0
* 2x - 3 = 0
2x = 3
x = 3/2
* 6 - 2x = 0
2x = 2
x = 1
Vậy x = 3/2 hoặc x = 1
19) (3x -1)(-1/2x + 5) = 0
3x - 1 = 0 hoặc -1/2x + 5 = 0
*3x - 1 = 0
3x = 1
x = 1/3
* -1/2x + 5 = 0
-1/2x = -5
x = 10
Vậy x = 1/3 hoặc x= 10
20) -2/7 < x/3 < 11/4 (x ∈ Z)
$\frac{(-2).12}{7.12}$ < $\frac{28.x}{28.3}$ < $\frac{21.11}{21.4}$
$\frac{-24}{84}$ < $\frac{28.x}{84}$ < $\frac{231}{84}$
-24 < 28x < 231
$\frac{-24}{28}$ < x < $\frac{231}{28}$
Vì $\frac{-24}{28}$ ≈ -0,9
$\frac{231}{28}$ ≈ 8,3
Nên -0,9 < x < 8,3
Mà x ∈ Z nên x ∈ {0; 1; 2; 3; 4; 5; 6; 7; 8}
