Có \(\frac{x}{4}=\frac{21}{28}\)

\(\Rightarrow x=\frac{21}{28}.4\)

\(\Rightarrow x=3\)

Vậy x=3

b, \(\frac{2x}{5}=\frac{-24}{10}\)

\(\frac{\Rightarrow2x}{5}=-\frac{12}{5}\)

\(\Rightarrow2x=-\frac{12}{5}.5\)

\(\Rightarrow2x=-12\)

\(\Rightarrow x=-12:2\)

\(\Rightarrow x=-6\)

Vậy x=-6

a) \(\frac{x}{4}=\frac{21}{28}\)

\(\Rightarrow\frac{x}{4}=\frac{3}{4}\)

\(\Rightarrow x=3\)

b)\(\frac{2x}{5}=\frac{-24}{10}\)

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

\(\Rightarrow2x=-12\)

\(\Rightarrow x=-6\)

\(a.\frac{x}{4}=\frac{21}{28}\)

\(\Rightarrow\frac{x}{4}=\frac{3}{4}\)

\(\Rightarrow x=3\)

Vậy x = 3

\(b.\frac{2x}{5}=-\frac{24}{10}\)

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

\(\Rightarrow2x=-12\)

\(\Rightarrow x=-6\)

Vậy x = - 6

a) x= 3

b) x= ko bít hjhj

nhưng bạn có thể giải ra dc ko

\(a,\frac{x}{4}=\frac{21}{28}\Rightarrow\)\(x.28=4.21\Rightarrow x.28=84\Rightarrow x=84:28\Rightarrow\)\(x=3\)

a) \(\frac{x}{4}=\frac{21}{28}\)

Ta có : \(\frac{21}{28}=\frac{21:7}{28:7}=\frac{3}{4}\)

\(\Rightarrow\frac{x}{4}=\frac{3}{4}\Rightarrow x=3\)

Vậy x = 3

b) \(\frac{2x}{5}=-\frac{24}{10}\)

Ta có : \(\frac{-24}{10}=\frac{-12}{5}\)

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

=> \(2x=-12\Rightarrow x=-6\)

Vậy x = - 6

mn. giúp em nha 

\(a.\dfrac{6}{5}=\dfrac{18}{x}\Rightarrow x=\dfrac{18\cdot5}{6}=15\\ \text{Vậy}\text{ }x=15.\)

\(b.\dfrac{3}{4}=\dfrac{-21}{x}\Rightarrow x=\dfrac{-21\cdot4}{3}=28\\ \text{ }\text{ }\text{ }\text{ }\text{Vậy }x=28.\)

\(c.\dfrac{x}{4}=\dfrac{21}{28}\Rightarrow x=\dfrac{21\cdot4}{28}=3\\ \text{Vậy }x=3.\)

\(d.\dfrac{-8}{2x}=\dfrac{3}{-9}\Rightarrow x=\dfrac{-8\cdot\left(-9\right)}{3}:2=12\\ \text{Vậy }x=12.\)

\(e.\dfrac{-4}{11}=\dfrac{x}{22}=\dfrac{40}{z}\\ \Rightarrow x=\dfrac{-4\cdot22}{11}=-8\\ \Rightarrow z=\dfrac{22\cdot40}{-8}=-110\\ \text{Vậy }x=-8;z=-110.\)

\(f.\dfrac{-3}{4}=\dfrac{x}{20}=\dfrac{21}{y}\\ \Rightarrow x=\dfrac{-3\cdot20}{4}=-15\\ \Rightarrow y=\dfrac{21\cdot20}{-15}=-28\\ \text{Vậy }x=-15;y=-28.\)

\(g.\dfrac{-4}{8}=\dfrac{x}{-10}=\dfrac{-7}{y}=\dfrac{z}{-24}\\ \Rightarrow x=\dfrac{-4\cdot\left(-10\right)}{8}=5\\ \Rightarrow y=\dfrac{-7\cdot\left(-10\right)}{5}=14\\ \Rightarrow z=\dfrac{-7\cdot\left(-24\right)}{14}=12\\ \text{Vậy }x=5;y=14;z=12.\)

\(h.\dfrac{x}{4}=\dfrac{9}{x}\\ \Rightarrow x\cdot x=9\cdot4\\ \Rightarrow x\cdot x=36\\ \Rightarrow x\cdot x=6\cdot6\\ \text{Vậy }\text{cả hai }x=6.\)

\(\frac{2x}{5}=-\frac{24}{10}\)

=> \(\frac{2x}{5}=-\frac{12}{5}\)

=> 2x = -12

=> x = -12  : 2

=> x = -6

\(\frac{x}{4}=\frac{21}{28}\)

=> \(\frac{x}{4}=\frac{3}{4}\)

=> x = 3

\(\frac{-24}{10}.\frac{x}{4}=\frac{-6}{10}.x=\frac{-3}{5}.x=\frac{21}{28}=\frac{3}{4}\)

\(\Rightarrow x=\frac{3}{4}:\frac{-3}{5}=\frac{-5}{4}\)

a)\(\frac{x}{4}=\frac{21}{28}\)

x . 28 = 4.21

x.28 = 84

x = 84:28

x = 3

Vậy x = 3

b)\(\frac{2x}{5}=\frac{-24}{10}\)

2x.10=5.(-24)

2x.10= -120

2x = -120 :10

2x= -12

x= -12 : 2

x= -6

Vậy x = -6

a,\(\frac{x}{4}\)=\(\frac{21}{28}\)

\(\Leftrightarrow\)28x=21.4

\(\Leftrightarrow\)28x=84

\(\Leftrightarrow\)x=3

Vậy x=3

b, \(\frac{2x}{5}\)=\(\frac{-24}{10}\)

\(\Leftrightarrow\)2x.10=-24.5

\(\Leftrightarrow\)20x=-120

\(\Leftrightarrow\)x=-6

Vậy x=-6

Chúc bn hk tốt

a/          5x +y -2x = 28 => 3x +y = 28

       x/10 = y/6 = z/21 = 3x /30= y/6 = 3x +y  /  36 = 28 /36 = 7/9

=> x= 70/9 ; y = 14/3 ; z= 49/3

b/

          x/3 = y/4 => x/15 = y/20 [1]

        y/5 = z/7 => y/20 = z/28  [2]

Từ [1] và [2] => x/15 = y/20 = z/28 = 2x /30 = 3y/60 = z/28 = [2x +3y - z] / [30+60-28]= 124 /62 = 2

=> x= 2 .15 = 30 ; y = 2x20 = 40 ; z= 2 . 28= 56

\((x-6)(3x-9)>0\)
TH1:
\(\orbr{\begin{cases}x-6< 0\\3x-9< 0\end{cases}}\)\(\Rightarrow\orbr{\begin{cases}x< 6\\x< 3\end{cases}}\)\(\Rightarrow x< 3\)
TH2:
\(\orbr{\begin{cases}x-6>0\\3x-9>0\end{cases}}\)\(\Rightarrow\orbr{\begin{cases}x>6\\x>3\end{cases}}\)\(\Rightarrow x>6\)
Vậy \(x< 3\) hoặc \(x>6\)thì \((x-6)(3x-9)>0\)
Học tốt!

20.
\((2x-1)(6-x)>0\)
TH1:
\(\orbr{\begin{cases}2x-1>0\\6-x>0\end{cases}\Rightarrow\orbr{\begin{cases}x< \frac{1}{2}\\x< 6\end{cases}}\Rightarrow x< 6}\)
TH2
\(\orbr{\begin{cases}2x-1< 0\\6-x< 0\end{cases}\Rightarrow\orbr{\begin{cases}x>\frac{1}{2}\\x>6\end{cases}}\Rightarrow x>\frac{1}{2}}\)
Vậy \(x< 6\)hoặc \(x>\frac{1}{2}\)thì \((2x-1)(6-x)>0\)
 

21.
\((2-x)(x+7)< 0\)
TH1.
\(\orbr{\begin{cases}2-x>0\\x+7< 0\end{cases}\Rightarrow\orbr{\begin{cases}x< 2\\x>-7\end{cases}}\Rightarrow-7< x< 2}\)
TH2.
\(\orbr{\begin{cases}2-x< 0\\x+7>0\end{cases}\Rightarrow\orbr{\begin{cases}x>2\\x< -7\end{cases}}\Rightarrow2< x< -7}\)(vô lí)
Vậy \(-7< x< 2\) thì \((2-x)(x+7)< 0\)
 

a) \(\frac{x}{4}=\frac{21}{28}\Rightarrow\frac{x}{4}=\frac{3}{4}\Rightarrow x=3\)

Vậy x = 3

b) \(\frac{2x}{5}=\frac{-24}{10}\Rightarrow\frac{2x}{5}=\frac{-12}{5}\Rightarrow2x=-12\Rightarrow x=-6\)

Vậy x = -6

\(a)\frac{x}{4}=\frac{21}{28}\)

\(\Rightarrow x\cdot28=4\cdot21\)

\(\Rightarrow x\cdot28=84\)

\(\Rightarrow x=3\)

\(b)\frac{2x}{5}=\frac{-24}{10}\)

Rút gọn : \(\frac{2x}{5}=\frac{-12}{5}\)

\(\Rightarrow2x=-12\)

\(\Rightarrow x=(-12)\div2=-6\)

a, \(\frac{x}{4}=\frac{21}{28}\)

Ta có : x . 28 = 4. 21

                  x = \(\frac{21.4}{28}\)

                  x = 3

b, \(\frac{2x}{5}=\frac{-24}{10}\)

Ta có : 2x . 10 = 5 . (-24)

                   2x = \(\frac{5.\left(-24\right)}{10}\)

                   2x = -12

                     x = -12 : 2

                     x = -6