x = 3

x = 3

\(120-\left(3x-9\right)=1440:12\\ \Rightarrow120-\left(3x-9\right)=120\\ \Rightarrow3x-9=120-120\\ \Rightarrow3x-9=0\\ \Rightarrow3x=0+9\\ \Rightarrow3x=9\\ \Rightarrow x=9:3\\ \Rightarrow x=3\)

a. 1440 : [ 120 - (3x + 9 ) ] = 120

                120 - (3x + 9)    = 1440 : 120 = 12

                         3x + 9      = 120 - 12 = 108

                         3x            = 108 - 9 = 99

                           x            = 99 : 3 = 33

b. 120 + [ ( 999 - 9x ) : 60 ] . 24 = 480

             [ ( 999 - 9x ) : 60 ] . 24 = 480  - 120 = 360

               ( 999 - 9x ) : 60          = 360 : 24 = 15 

                ( 999 - 9x )                = 15 . 60 = 900

                           9x                  = 999 - 900 = 99

                             x                  = 99 : 9 = 11

Câu 1: 

=>5x=15

hay x=3

Câu 2: 

\(\Leftrightarrow120-\left(3x+9\right)=12\)

=>3x+9=108

=>3x=99

hay x=33

\(\dfrac{4-x}{-5}=\dfrac{-5}{4-x}\)
\(\Leftrightarrow\left(4-x\right)\left(4-x\right)=-5\times-5\)
\(\Rightarrow\left(4-x\right)^2=25\)
\(\Rightarrow\left[{}\begin{matrix}4-x=5\\4-x=-5\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=-1\\x=9\end{matrix}\right.\)

\(\dfrac{4-x}{-5}=\dfrac{-5}{4-x}\)

\(\Rightarrow\left(4-x\right).\left(4-x\right)=\left(-5\right).\left(-5\right)\)

\(\Rightarrow\left(4-x\right)^2=25\)

\(\Rightarrow\left(4-x\right)^2=5^2\)

\(\Rightarrow4-x=\pm5\)

\(\Rightarrow\left[{}\begin{matrix}4-x=5\\4-x=-5\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}x=4-5\\x=4-\left(-5\right)\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}x=-1\\x=9\end{matrix}\right.\)

=>(4-x)^2=25

=>(x-4)^2=25

=>x-4=5 hoặc x-4=-5

=>x=-1 hoặc x=9

Bài 1:

a, 96 \(⋮x=>x\inư\left(96\right)\)

b, \(2^x.15+2^x.17=4^{30}\)

\(2^x\left(15+17\right)=4^{30}\)

\(2^x.32=4^{30}\)

\(2^x.2^5=2^{60}\)

\(2^x=2^{60}:2^5\)

\(2^x=2^{55}\)

=> x = 55

\(a,0,5x-\frac{2}{3}x=\frac{7}{12}\Rightarrow\frac{1}{2}x-\frac{2}{3}x=\frac{7}{12}\)

\(\Rightarrow x\left(\frac{1}{2}-\frac{2}{3}\right)=\frac{7}{12}\Rightarrow x\cdot\left(\frac{3}{6}-\frac{4}{6}\right)=\frac{7}{12}\)

\(\Rightarrow x\cdot\left(-1\right)=\frac{7}{12}\Rightarrow x=\frac{7}{12}:\left(-1\right)=\frac{7}{-12}\)

\(c,\frac{\left(x-5\right)}{12}\cdot\frac{9}{29}=\frac{-6}{29}\Rightarrow\frac{\left(x-5\right)}{12}=\frac{-6}{29}:\frac{9}{26}\)

\(\frac{\Rightarrow\left(x-5\right)}{12}=\frac{-6}{9}=\frac{-2}{3}\Rightarrow x-5=-\frac{2}{3}\cdot12\)

\(\Rightarrow x-5=\frac{-24}{3}=-8\Rightarrow x=-8+5=-3\)

\(a,0,5x-\frac{2}{3}x=\frac{7}{12}\)

\(\Rightarrow\frac{1}{2}x-\frac{2}{3}x=\frac{7}{12}\)

\(\Rightarrow-\frac{1}{6}x=\frac{7}{12}\)

\(\Rightarrow x=-\frac{7}{2}\)

\(c,\frac{x-5}{12}\cdot\frac{9}{29}=-\frac{6}{29}\)

\(\Rightarrow\frac{x-5}{12}=-\frac{2}{3}\)

\(\Rightarrow x-5=12.\left(-\frac{2}{3}\right)\)

\(\Rightarrow x-5=-8\)

\(\Rightarrow x=-3\)

b)\(\frac{-8}{17}+\frac{5}{17}< \frac{x}{17}< \frac{-6}{17}+\frac{9}{17}\)\(\left(x\in Z\right)\)

\(\Rightarrow\frac{-3}{17}< \frac{x}{17}< \frac{3}{17}\)

\(\Rightarrow-3< x< 3\)

\(\Rightarrow x\in\left\{-2;-1;0;1;2\right\}\)

Vậy \(x\in\left\{-2;-1;0;1;2\right\}\)

\(4^x+4^{x+3}=4160\)

\(\Leftrightarrow4^x.\left(1+4^3\right)=4160\)

\(\Leftrightarrow4^x.65=4160\)

\(\Leftrightarrow4^x=4160:65=64\)

\(\Rightarrow x=3\)

x=9,16515139?

4 /x=y/21=28/49

vì 21.4/12=7 nên x=7

    28.21/49 =12 nên y=12 (công thức nhân chéo)

vậy ,x=7 ;y=12