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

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

Bài 1:

a) \(=\dfrac{8}{15}\left(\dfrac{7}{13}+\dfrac{6}{13}\right)=\dfrac{8}{15}.1=\dfrac{8}{15}\)

b) \(=\dfrac{3.3-7-2.4}{12}=-\dfrac{6}{12}=-\dfrac{1}{2}\)

Bài 2:

 \(\dfrac{x}{2,7}=-\dfrac{2}{3,6}\Rightarrow x=\dfrac{\left(-2\right).2,7}{3,6}\Rightarrow x=-\dfrac{3}{2}\)

Bài 3:

\(\dfrac{x}{2}=\dfrac{y}{5}=\dfrac{x+y}{2+5}=-\dfrac{21}{7}=-3\)

\(\Rightarrow\left\{{}\begin{matrix}x=\left(-3\right).2=-6\\y=\left(-3\right).5=-10\end{matrix}\right.\)

a, -1

b, 2

Câu a) x là 3/8 nha.

Câu b) x là 1/3

Giải ra dùm vs 

a. (x+3)-21=-21

=> x+3=-21+21

=> x+3=0

=> x=0-3

Vậy x=-3.

b. |x|-5=7

=> |x|=7+5

=> |x|=12

=> x = 12 hoặc x=-12.

Vậy x \(\in\){-12; 12}

c. |3-x|+2=13

=> |3-x|=13-2

=> |3-x|=11

 +) 3-x=11

=> x=3-11

=> x=-8

 +) 3-x=-11

=> x=3-(-11)

=> x=3+11

=> x=14

Vậy x \(\in\) {-8; 14}.

a , (x+3) -21=-21                               b ,|x|-5=7                                    c,  |3-x|+2=13

    x+3=0         x= -3                              |x|=12       x=12                          |3-x|=11

                                                                             =-12                         3-x=11          x= -8

                                                                                                             3-x=-11          =14

a) => 1/2 - 5/6 = x + 1/3 => -1/3 = x + 1/3 => x = 2/3

b) => -21/13x = -2/3 - 1/3 => -21/13x = -1 => x = 13/21

c) => GTTD 2x +1 = 1/8 => 2x + 1 = 1/8 hoac 2x + 1 = -1/8 => 2x = 1/8 -1 hoac 2x = -1/8 - 1 => 2x = -7/8 hoac 2x = -9/8 => x = -7/16 hoac x = -9/16