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

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

b. Câu hỏi của Nguyen Hai Bang - Toán lớp 7 - Học toán với OnlineMath

= 203/24

\(\left(\frac{136}{15}-\frac{28}{5}+\frac{62}{10}\right)\cdot\frac{21}{24}\)

=

\(=\left(\frac{272}{30}+-\frac{168}{30}+\frac{186}{30}\right).\frac{7}{8}\)\(=\frac{29}{3}.\frac{7}{8}=\frac{203}{24}\)

\(\left(\frac{136}{15}-\frac{28}{5}+\frac{62}{10}\right).\frac{21}{24}\)

\(=\left(\frac{136}{15}-\frac{28}{5}+\frac{31}{5}\right).\frac{21}{24}\)

\(=\left[\frac{136}{15}-\left(\frac{28}{5}-\frac{31}{5}\right)\right].\frac{21}{24}\)

\(=\left(\frac{136}{15}+\frac{3}{5}\right).\frac{21}{24}\)

\(=\frac{29}{3}.\frac{21}{24}\)

\(=\frac{203}{24}\)

Study well ! >_<

\(\frac{203}{24}\)

\(a,\frac{x}{10}=\frac{y}{6}=\frac{z}{21}=\frac{5x+y-2z}{50+6-42}=\frac{28}{14}=2\)

\(\frac{x}{10}=2\Rightarrow x=10.2=20\)

\(\frac{y}{6}=2\Rightarrow y=2.6=12\)

\(\frac{z}{21}=2\Rightarrow z=21.2=42\)

\(d,\frac{x}{2}=\frac{y}{3}=k\)\(\Rightarrow x=2k;y=3k\)

\(\Rightarrow ab=2k.3k=6k^2=54\)

\(\Rightarrow k^2=9\Leftrightarrow k=3\)

\(\frac{x}{2}=3\Rightarrow x=6\)

\(\frac{y}{3}=3\Rightarrow y=9\)

a) Áp dụng t/c của dãy tỉ số bằng nhau, ta có:

\(\frac{x}{10}=\frac{y}{6}=\frac{z}{21}\) => \(\frac{5x}{50}=\frac{y}{6}=\frac{2z}{42}=\frac{5x+y-2z}{50+6-42}=\frac{28}{14}=2\)

=> \(\hept{\begin{cases}\frac{x}{10}=2\\\frac{y}{6}=2\\\frac{z}{21}=2\end{cases}}\)   =>  \(\hept{\begin{cases}x=2.10=20\\y=2.6=12\\z=2.21=42\end{cases}}\)

Vậy x = 20; y = 12; z = 42

b) Ta có: \(\frac{x}{3}=\frac{y}{4}\) => \(\frac{x}{15}=\frac{y}{20}\)

          \(\frac{y}{5}=\frac{z}{7}\)  => \(\frac{y}{20}=\frac{z}{28}\)

=> \(\frac{x}{15}=\frac{y}{20}=\frac{z}{28}\)

Áp dụng t/c của dãy tỉ số bằng nhau, ta có:

\(\frac{x}{15}=\frac{y}{20}=\frac{z}{28}\)=> \(\frac{2x}{30}=\frac{3y}{60}=\frac{z}{28}=\frac{2x+3y-z}{30+60-28}=\frac{125}{62}=\frac{125}{62}\)

=> \(\hept{\begin{cases}\frac{x}{15}=\frac{125}{62}\\\frac{y}{20}=\frac{125}{62}\\\frac{z}{28}=\frac{125}{62}\end{cases}}\)  =>  \(\hept{\begin{cases}x=\frac{125}{62}.15=\frac{1875}{62}\\y=\frac{125}{62}.20=\frac{1250}{31}\\z=\frac{125}{62}.28=\frac{1750}{31}\end{cases}}\)

Vậy ...

b) \(\frac{x}{3}=\frac{y}{4}\Rightarrow\frac{2x}{30}=\frac{3y}{60};\frac{y}{5}=\frac{z}{7}\Rightarrow\frac{3y}{60}=\frac{z}{28}\) 

\(\Rightarrow\frac{2x}{30}=\frac{3y}{60}=\frac{z}{28}\) 

Áp dụng dãy tỉ số bằng nhau:

đến đây dễ rồi bạn tự lm tiếp nhé

c) \(\frac{2x}{3}=\frac{3y}{4}=\frac{4z}{5}\) 

\(\Rightarrow\frac{x}{\frac{3}{2}}=\frac{y}{\frac{4}{3}}=\frac{z}{\frac{5}{4}}\) 

Áp dụng dãy tỉ số bằng nhau:

.............

d) Ta có:

 \(xy=54\Rightarrow x=\frac{54}{y}\) 

\(\frac{x}{2}=\frac{\frac{54}{y}}{2}=54.\frac{2}{y}=\frac{108}{y}\) 

Ta lại có:\(\frac{x}{2}=\frac{y}{3}\Rightarrow\frac{108}{y}=\frac{y}{3}\Rightarrow y^2=324\Leftrightarrow y=18\)  

thay vào \(\frac{x}{2}=\frac{y}{3}\Leftrightarrow\frac{x}{2}=\frac{18}{3}\Leftrightarrow x=12\) 

Vậy.....