Cậu mới thi viuolympic sao mk cũng mới đăng

đặt \(\left(\frac{1}{2.5}+\frac{1}{5.8}+\frac{1}{8.11}+....+\frac{1}{65.68}\right)\)là A

Ax=\(\frac{19}{68}+\frac{7}{34}=\frac{33}{68}\)

3A=\(3.\left(\frac{1}{2.5}+\frac{1}{5.8}+\frac{11}{8.11}+...+\frac{1}{65.68}\right)\)

3A=\(\frac{3}{2.5}+\frac{3}{5.8}+\frac{3}{8.11}+...+\frac{3}{65.68}\)

3A=\(\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+...+\frac{1}{65}-\frac{1}{68}\)

3A=\(\frac{1}{2}-\frac{1}{68}=\frac{33}{68}\)

A=33/68:3=11/68

\(\Rightarrow\)33/68:11/68=3

vậy x= 3

Trả lời giúp mk nha mk cho 3 h lun nói thietj ó

ủng hộ mình nha

  \(\Rightarrow A=\frac{1}{3}\left(\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+....+\frac{1}{65}-\frac{1}{68}\right)\)

\(\Rightarrow A=\frac{1}{3}\left(\frac{1}{2}-\frac{1}{68}\right)=\frac{1}{2}\left(\frac{34}{68}-\frac{1}{68}\right)=\frac{1}{2}.\frac{33}{68}=\frac{33}{136}\)

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tich mik minh tich lai

=1/5-1/x(x+3)

=x=98

<=> \(\frac{1}{3}\cdot\left(\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+...+\frac{1}{x}-\frac{1}{x+3}\right)=\frac{101}{1540}\)

<=>\(\frac{1}{3}\cdot\left(\frac{1}{5}-\frac{1}{x+3}\right)=\frac{101}{1540}\)

<=>\(\frac{1}{5}-\frac{1}{x+3}=\frac{101}{1540}\cdot3=\frac{303}{1540}\)

<=>\(\frac{1}{x+3}=\frac{1}{5}-\frac{303}{1540}=\frac{1}{308}\)

<=>\(x+3=308\)

<=>\(x=305\)

a) 100 - 7 . (x - 5) = 58

7. (x - 5) = 100 - 58

7. (x - 5) = 42

x - 5 = 42 : 7

x - 5 = 6

x = 6 + 5 

x = 11

b)\(x+\frac{1}{3}=\frac{7}{26}.\frac{13}{6}\)

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

\(x=\frac{3}{12}=\frac{1}{4}\)

Thiếu câu c) bạn ơi!

ok trả lời liền

Ta có : A = 1/ 2.5 + 1/ 5.8 + 1/ 8.11 + ... + 1/ (3n-1).(3n+2) .

              = 1/2 - 1/5 + 1/5 - 1/8 + 1/8 - 1/11 + ... + 1/ 3n-1 - 1/ 3n+2 .

              = 1/2 - 1/ 3n+2 .

              = 3n + 2 - 2 / 2 .( 3n+2 ) .

             = 3n / 2.(3n+2) .

bai 1:\(\frac{1}{3}\left(\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+...+\frac{1}{65}-\frac{1}{68}\right)x-\frac{7}{34}=\frac{19}{68}\)

=\(\frac{1}{3}\left(\frac{1}{2}-\frac{1}{68}\right)x-\frac{7}{34}=\frac{19}{68}\)

\(\Rightarrow\)x=3

bai2:từ giả thiết \(\frac{ab}{bc}=\frac{a}{c}=\frac{-1}{2}va\frac{ab}{ac}=\frac{b}{c}=\frac{3}{4}\)

hay \(\frac{a}{-2}=\frac{b}{3}=\frac{c}{4}\)

\(\Rightarrow\)\(\left(\frac{a}{-2}\right)^2=\frac{a}{-2}\times\frac{b}{3}=\frac{-6}{-6}=1\)

a=-2 (a<0)

\(\Rightarrow\)a=-2,b=3,c=4

x=1

a= -2

b=3

c=4