Vì \(\frac{-x}{21}=\frac{20}{7}\)suy ra -x . 7.=21. 20

x=\(\frac{21.20}{7}\)

x=60

mình đoán là : 60

hai bn đúng òi!

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

\(\frac{5}{6}=\frac{15}{24}\) 

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

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

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

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

CHÚC BẠN HỌC GIỎI

bài này dễ mà

=>(x-7)\(\times\)(\(\frac{1}{20}+\frac{1}{21}+\frac{1}{32}-\frac{1}{23}+\frac{1}{54}\))=0

Mà \(\frac{1}{20}+\frac{1}{21}+\frac{1}{32}-\frac{1}{23}+\frac{1}{54}\)\(\ne\)0

=>x-7=0

=>x=7

Vậy x=7

chúc bn học tốt. nhớ like cho mik nha

                                                                     Bài giải

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

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

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

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

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

Hoàng Thị Thanh Trúc

a,

\(\frac{x}{7}=\frac{6}{21}\Rightarrow x=\frac{6.7}{21}=2\)

b,

\(\frac{-5}{y}=\frac{20}{28}\Rightarrow y=\frac{\left(-5\right).28}{20}=-7\)

Ta có: \(x-\frac{20}{11\cdot13}-\frac{20}{13\cdot15}-...-\frac{20}{53\cdot55}=\frac{3}{11}\)

\(\Leftrightarrow x-10\cdot\left(\frac{2}{11\cdot13}+\frac{2}{13\cdot15}+...+\frac{2}{53\cdot55}\right)=\frac{3}{11}\)

\(\Leftrightarrow x-10\cdot\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+...+\frac{1}{53}-\frac{1}{55}\right)=\frac{3}{11}\)

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

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

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

\(\Leftrightarrow x=\frac{3}{11}+\frac{8}{11}\)

\(\Leftrightarrow x=1\)

Vậy \(x=1\)thỏa mãn đề.