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Bài 1 :
S = \(\frac{6}{2.5}+\frac{6}{5.8}+...+\frac{6}{29.32}\)
= 2 . \(\left(\frac{3}{2.5}+\frac{3}{5.8}+...+\frac{3}{29.32}\right)\)
= 2 . \(\left(\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+...+\frac{1}{29}-\frac{1}{32}\right)\)
= 2 . \(\left(\frac{1}{2}-\frac{1}{32}\right)\)= ....
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\text{-7129+1478+7129+(-1479)}\)
\(=\text{-7129+7129+1478+(-1479)}\)
\(=0-1=-1\)
\(\text{|-5|.(-7)+4.(-9)}\)
\(=\text{5.(-7)+4.(-9)}\)
\(=\left(-35\right)+\left(-36\right)=-71\)
![](https://rs.olm.vn/images/avt/0.png?1311)
x.\(\left(\frac{3}{2.5}+\frac{3}{5.8}+\frac{3}{8.11}+\frac{3}{11.14}+\frac{3}{14.17}\right)=-1\frac{3}{5}\)
x.\(\left(\frac{5-2}{2.5}+\frac{8-5}{5.8}+\frac{11-8}{8.11}+\frac{14-11}{11.14}+\frac{17-14}{14.17}\right)=\frac{-8}{5}\)
x.\(\left(\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}+\frac{1}{14}-\frac{1}{17}\right)=\frac{-8}{5}\)
x.\(\left(\frac{1}{2}-\frac{1}{17}\right)=\frac{-8}{5}\)
x.\(\left(\frac{17}{34}-\frac{2}{34}\right)=\frac{-8}{5}\)
x.\(\frac{15}{34}=\frac{-8}{5}\)
x\(=\frac{-8}{5}:\frac{15}{34}\)
x\(=\frac{-8}{5}.\frac{34}{15}\)
x\(=\frac{-272}{75}\)
Vậy x\(=\frac{-272}{75}\)
a) 2/2.5 + 2/5.8 + 2/8.11 + ... + 2/x(x+3) = 7/23
3/2.5 + 3/5.8 + 3/8.11 + ... + 3/x(x+3) = 21/46
1/2 - 1/5 + 1/5 - 1/8 + 1/8 - 1/11 + ... + 1/x - 1/x+1 = 21/46
1/2 - 1/x+1 = 21/46
=> 1/x+1 = 1/23
=> x + 1 = 23
=> x = 22
Vậy x = 22.
b) 3/4 . x - 1/5 = 7/4 . x + 11/5
3/4 . x - 7/4 . x = 1/5 + 11/5
x (3/4 - 7/4) = 12/5
-x = 12/5
x = -12/5
Vậy x = -12/5.