\(\frac{-x}{21}=\frac{20}{7}\)

\(\Rightarrow-x.7=21.20\)

\(\Rightarrow-x.7=420\)

\(\Rightarrow-x=420:7\)

\(\Rightarrow-x=-60\)

\(\frac{-x}{21}=\frac{20}{7}\)

\(\Rightarrow-x.7=20.21\)

\(\Rightarrow-7x=420\)

\(\Rightarrow x=420:-7\)

\(\Rightarrow x=-60\)

Vậy \(x=-60\)

Chúc bạn học tốt !!! 

\(\frac{-x}{21}=\frac{20}{7}\)

\(\Leftrightarrow-x.7=20.21\)

\(\Leftrightarrow-x.7=210\)

\(\Leftrightarrow-x=30\)

\(\Leftrightarrow x=30\)

                                                                     Bài giải

\(-\frac{20}{21}< x+\frac{4}{7}< \frac{7}{12}\)

\(-\frac{20}{21}-\frac{4}{7}< x+\frac{4}{7}-\frac{4}{7}< \frac{7}{12}-\frac{4}{7}\)

\(-\frac{32}{21}< x< \frac{1}{84}\)

\(-1,5...< x< 0,01...\)

\(\Rightarrow\text{ }x=-1\)

\(\frac{x}{7}=\frac{6}{21}\)

\(\Rightarrow x=\frac{7.6}{21}\)

\(\Rightarrow x=2\)

\(-\frac{5}{y}=\frac{20}{28}\)

\(\Rightarrow y=\frac{-5.28}{20}\)

\(\Rightarrow y=-7\)

a) \(\left(3\frac{1}{2}-2x\right).3\frac{1}{3}=7\frac{1}{3}\)

 \(\left(\frac{7}{2}-2x\right).\frac{10}{3}=\frac{22}{3}\)

  \(\frac{7}{2}-2x=\frac{11}{5}\)

              \(2x=\frac{13}{10}\)

                \(x=\frac{13}{20}\)

Vậy ...

b) \(\frac{4}{9}x=\frac{9}{8}-0,125\)

   \(\frac{4}{9}x=1\)

         \(x=\frac{9}{4}\)

Vậy...

mình ko ghi đầu bài đâu nhé

a, 7/2-2x=22/3:10/3

   7/2-2x=11/5

  2x=7/2-11/5

  2x=13/10

  x=13/10:2

 x=13/20

Bài 1:

a)  \(x-\frac{20}{11.13}-\frac{20}{13.15}-...-\frac{20}{53.55}=\frac{3}{11}\)

\(x-\left(\frac{20}{11.13}+\frac{20}{13.15}+...+\frac{20}{53.55}\right)=\frac{3}{11}\)

\(x-\frac{20}{2}.\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+...+\frac{1}{53}-\frac{1}{55}\right)=\frac{3}{11}\)

\(x-10.\left(\frac{1}{11}-\frac{1}{55}\right)=\frac{3}{11}\)

\(x-10\cdot\frac{4}{55}=\frac{3}{11}\)

\(x-\frac{8}{11}=\frac{3}{11}\)

\(x=1\)

b) \(\frac{1}{21}+\frac{1}{28}+\frac{1}{36}+...+\frac{2}{x.\left(x+1\right)}=\frac{2}{9}\)

\(\frac{2}{42}+\frac{2}{56}+\frac{2}{72}+...+\frac{2}{x.\left(x+1\right)}=\frac{2}{9}\)

\(\frac{2}{6.7}+\frac{2}{7.8}+\frac{2}{8.9}+...+\frac{2}{x.\left(x+1\right)}=\frac{2}{9}\)

\(2.\left(\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+...+\frac{1}{x}-\frac{1}{x+1}\right)=\frac{2}{9}\)

\(2.\left(\frac{1}{6}-\frac{1}{x+1}\right)=\frac{2}{9}\)

\(\frac{1}{6}-\frac{1}{x+1}=\frac{1}{9}\)

\(\frac{1}{x+1}=\frac{1}{18}\)

=> x + 1 =18

x = 17

bài 2 ko bk lm, xl nha

mk cảm ơn bn nha

\(2x+\frac{7}{6}+\frac{13}{12}+\frac{21}{20}+\frac{31}{30}+\frac{43}{42}+\frac{57}{56}+\frac{73}{72}+\frac{91}{90}=10\)
=> \(2x+\frac{6+1}{6}+\frac{12+1}{12}+....+\frac{90+1}{90}=10\)
=> \(2x+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{90}+10=10\)
=> \(2x+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{90}=0\)
=>\(2x+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{9.10}=0\)
=>\(2x+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{9}-\frac{1}{10}=0\)
=> \(2x-\frac{1}{10}=0\)
=>2x=\(\frac{1}{10}\)=> x=1/20
 

mình có bị nhầm chỗ dấu suy ra thứ 3. đáng lẽ ra biểu thức đó cộng 8 chứ k phải cộng 10 do mình sơ ý nên bạn hãy sủa lại chỗ ấy

đây nhá tại đánh máy lâu quá nên gửi hình ^^

\(\frac{5}{6}=\frac{15}{24}\) 

\(\frac{4}{7}=\frac{12}{21}\)

\(\frac{3}{9}=\frac{7}{21}\)

\(\frac{-5}{-4}=\frac{20}{16}\)

5/6 = x/24 = 5*24 = 6*x = 120 = 6*x suy ra x= 120:6=20

4/7 = 12/x = 4*x = 7*12 = 4*x = 84 suy ra x= 84:4=21

3/x = 7/21 = 3/x = 1/3 = 3*3 = 1*x = 9 = 1*x suy ra x=9:1=9

x/-4 = 20/16 = x/-4 = 5/4 = x*4 = -4*5 = x*4 = -20 suy ra x=-20:4=-5

a) \(\frac{5}{6}\)= \(\frac{x}{24}\)

\(\Rightarrow\)\(x.6=24.5\)

x = \(\frac{24\cdot5}{6}\)

x=20 

Vậy x = 20

b)\(\frac{4}{7}\)=\(\frac{20}{x}\)

\(\Rightarrow\)\(4\cdot x=20\cdot7\)

x= \(\frac{20\cdot7}{4}\)

x=35

Vậy x = 35

c)\(\frac{3}{x}\)=\(\frac{7}{21}\)

\(\Rightarrow\)\(x\cdot7=3\cdot21\)

x= \(\frac{3\cdot21}{7}\)

x= 9 

Vậy x =9

d) \(\frac{x}{-4}\)=\(\frac{20}{16}\)

\(\Rightarrow\)\(x\cdot16=-4\cdot20\)

x= \(\frac{\left(-4\right)\cdot20}{16}\)

x =16 

Vậy x = 16

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