a) x = 407

b) x = 13

c) x = 5

d) x = 3

a, \(\dfrac{x}{3}=\dfrac{8}{12};12:3=4\)\(;8:4=2\)

⇒\(x=\dfrac{2}{3}\)

chờ mk tìm cách giải các  ý khác đã

Bài 15:

a: 2x+x=45

=>3x=45

=>\(x=\frac{45}{3}=15\)

b: 2x+7x=918

=>\(x\cdot\left(7+2\right)=918\)

=>9x=918

=>\(x=\frac{918}{9}=102\)

c: 2x+3x=60+5

=>5x=65

=>\(x=\frac{65}{5}=13\)

d: \(11x+22x=33\cdot2\)

=>33x=66

=>\(x=\frac{66}{33}=2\)

Bài 14:

a: (12-x)(2-x)=0

=>\(\left[\begin{array}{l}12-x=0\\ 2-x=0\end{array}\right.\Rightarrow\left[\begin{array}{l}x=12\\ x=2\end{array}\right.\)

b: (x-33)(11-x)=0

=>\(\left[\begin{array}{l}x-33=0\\ 11-x=0\end{array}\right.\Rightarrow\left[\begin{array}{l}x=33\\ x=11\end{array}\right.\)

c: (21-x)(12-x)=0

=>\(\left[\begin{array}{l}21-x=0\\ 12-x=0\end{array}\right.\Rightarrow\left[\begin{array}{l}x=21\\ x=12\end{array}\right.\)

d: (50-x)(x-150)=0

=>\(\left[\begin{array}{l}50-x=0\\ x-150=0\end{array}\right.\Rightarrow\left[\begin{array}{l}x=50\\ x=150\end{array}\right.\)

Bài 13:

a: (x-2)(x-3)=0

=>\(\left[\begin{array}{l}x-2=0\\ x-3=0\end{array}\right.\Rightarrow\left[\begin{array}{l}x=2\\ x=3\end{array}\right.\)

b: (x-3)(x-4)=0

=>\(\left[\begin{array}{l}x-3=0\\ x-4=0\end{array}\right.\Rightarrow\left[\begin{array}{l}x=3\\ x=4\end{array}\right.\)

c: (x-7)(6-x)=0

=>\(\left[\begin{array}{l}x-7=0\\ 6-x=0\end{array}\right.\Rightarrow\left[\begin{array}{l}x=7\\ x=6\end{array}\right.\)

d: (x-3)(x-13)=0

=>\(\left[\begin{array}{l}x-3=0\\ x-13=0\end{array}\right.\Rightarrow\left[\begin{array}{l}x=3\\ x=13\end{array}\right.\)

\(-12\left(x-5\right)+7\left(3-x\right)=15\)

\(\Rightarrow-12x+60+21-7x=15\)

\(\Rightarrow-12x-7x=15-21-60\)

\(\Rightarrow-19x=-66\)

\(\Rightarrow x=\frac{66}{19}\)

\(\left|2x-5\right|=12\)

\(\Leftrightarrow\orbr{\begin{cases}2x-5=12\\2x-5=-12\end{cases}}\)

\(\Leftrightarrow\orbr{\begin{cases}2x=17\\2x=-7\end{cases}}\)

\(\Leftrightarrow\orbr{\begin{cases}x=\frac{17}{2}\left(t/m\right)\\x=-\frac{7}{2}\left(t/m\right)\end{cases}}\)

Giải : 

\(\frac{x+1}{x-2}=\frac{3}{4}\)

\(\Rightarrow4.\left(x-1\right)=3.\left(x-2\right)\)

\(\Rightarrow4x-4=3x-6\)

\(\Rightarrow4x-4-3x+6=0\)

\(\Rightarrow x+2=0\)

\(\Rightarrow x=-2\)Không thỏa mãn => Không có giá trị x thỏa mãn đề bài 

\(\frac{2x-3}{x+1}=\frac{4}{7}\)

\(\Rightarrow7.\left(2x-3\right)=4.\left(x+1\right)\)

\(\Rightarrow14x-21-4x-4=0\)

\(\Rightarrow10x-25=0\)

\(\Rightarrow10x=25\)

\(\Rightarrow x=\frac{25}{10}=\frac{5}{2}\)

Giá trị trên thỏa mãn đầu bài

Các phần khác em làm tương tự nha

-12( x - 5 ) + 7(3 - x ) = 15

=> -12x +60 + 21 - 7x = 15

=> -12x - 7x + 60 + 21 = 15

=> -19x + 81 = 15

=> -19x = 15 - 81 = -66

=> x = 66/19

(15-7x).3=15+9

(15-7x).3=24

(15-7x)=24:3

(15-7x)=8

       7x=15-8

       7x=7

         x=7:7

       x=1

vậy x=1

( 15 - 7x ) . 3 - 9 = 15

( 15 - 7x ) . 3      = 15+9

( 15 - 7x ) . 3      = 24

 15 - 7x               = 24:3

 15 - 7x               = 8

        7x               = 15-8

        7x              =7

          x              = 7:7

          x              = 1

Vậy x=1