Giải các phương trình sau: 1. a) 7x+12=0. b) 12-6x=0. c) 12-6x=0. d)-2x+14=0 2. a) 3x +1=7x-11. b)2x+x+12=0. c). x-5=3-x. d) 7-3x=9-x e)5-3x=6x

1. a. 7x + 12 = 0

$\Leftrightarrow$ 7x = -12
$\Leftrightarrow$ x = $\frac{-12}{7}$ 

S = { $\frac{-12}{7}$ }

b. 12 – 6x = 0
$\Leftrightarrow$ 6( 2 – x ) = 0
$\Leftrightarrow$ 2 – x = 0
$\Leftrightarrow$ x = 2
S = { 2 }

c. 12 – 6x = 0
$\Leftrightarrow$ 6( 2 – x ) = 0
$\Leftrightarrow$ 2 – x = 0
$\Leftrightarrow$ x = 2
S = { 2 }

d. -2x + 14 = 0
$\Leftrightarrow$ -2( x – 7) = 0
$\Leftrightarrow$ x – 7 = 0
$\Leftrightarrow$ x = 7

Bài 2:
a. 3x + 1 = 7x – 11 
$\Leftrightarrow$ 3x – 7x = -11 -1 
$\Leftrightarrow$ -4x = -12
$\Leftrightarrow$ x = 3

b. 2x + x + 12 = 0
$\Leftrightarrow$ 3x = -12
$\Leftrightarrow$ x = -4

c. x – 5 = 3 – x
$\Leftrightarrow$ x + x = 3 + 5
$\Leftrightarrow$ 2x = 8
$\Leftrightarrow$ x = 4

d. 7 – 3x = 9 – x
$\Leftrightarrow$ -3x + x = 9 – 7
$\Leftrightarrow$ -2x = 2 
$\Leftrightarrow$   x = -1

e. 5 – 3x = 6x + 7
$\Leftrightarrow$ -3x – 6x = 7 – 5
$\Leftrightarrow$ -9x = 2
$\Leftrightarrow$ x=$\frac{-2}{9}$ 

f. 11 – 2x = x – 1
$\Leftrightarrow$ -2x -x = -1 – 11
$\Leftrightarrow$ -3x = -12
$\Leftrightarrow$ x = 4

g. 15 – 8x = 9 – 5x 
$\Leftrightarrow$ -8x + 5x = 9 – 15
$\Leftrightarrow$ 3x = -6
$\Leftrightarrow$ x = -2

h. 3 + 2x = 5 + 2x 
$\Leftrightarrow$ 2x – 2x = 5 – 3
$\Leftrightarrow$ 0 = 2 ( vô lí )