![](https://rs.olm.vn/images/avt/0.png?1311)
Hãy nhập câu hỏi của bạn vào đây, nếu là tài khoản VIP, bạn sẽ được ưu tiên trả lời.
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
C = 1/(9.10) - 1/(8.9) - 1/(7.8) - ... - 1/(2.3) - 1/(1.2)
= 1/9 - 1/10 - 1/8 + 1/9 - 1/7 + 1/8 - ... - 1/2 + 1/3 - 1 + 1/2
= 1/9 - 1/10 + 1/9 - 1
= 2/9 - 11/10
= -79/90
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\frac{1}{9.10}-\frac{1}{8.9}-\frac{1}{7.8}-\frac{1}{6.7}-\frac{1}{5.6}-\frac{1}{4.5}-\frac{1}{3.4}-\frac{1}{2.3}-\frac{1}{1.2}\)
\(=-\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}\right)+\frac{1}{9.10}\)
\(=-\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{8}-\frac{1}{9}\right)+\frac{1}{90}\)
\(=-\left(1-\frac{1}{10}\right)+\frac{1}{90}\)
\(=-\frac{9}{10}+\frac{1}{90}\)
= ...
bn tự tính nha!
![](https://rs.olm.vn/images/avt/0.png?1311)
1/9.10 - ( 1/1.2 + 1/2.3 +...+ 1/8 .9)
= 1/9 - 1/10 -( 1/1 - 1/2 + 1/2 - 1/3 +. ..+1/8 - 1 9/)
= 1/9 - 1/10 - ( 1 - 1/9 )
= 1/9 - 1/10 - 1 + 1/9
=-79/9
thang Tran làm cách giải đúng rồi, nhưng đáp số là -79/90 nha.
Dù sao cũng cám ơn bạn vì giúp mình hiểu bài.
![](https://rs.olm.vn/images/avt/0.png?1311)
bị nhầm xíu sửa lại
\(\frac{1}{9.10}-\frac{1}{8.9}-\frac{1}{7.8}-...-\frac{1}{2.3}-\frac{1}{1.2}=\frac{1}{9}-\frac{1}{10}-\frac{1}{8}+\frac{1}{9}-\frac{1}{7}+\frac{1}{8}-...-\frac{1}{2}+\frac{1}{3}-\frac{1}{1}+\frac{1}{2}\)
\(=\frac{2}{9}-\frac{1}{10}-\frac{1}{1}=\frac{20}{90}-\frac{9}{90}-\frac{90}{90}=-\frac{79}{90}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có: \(M=\frac{1}{9.10}-\frac{1}{8.9}-\frac{1}{7.8}-\frac{1}{6.7}-\frac{1}{5.6}-...-\frac{1}{1.2}\)
\(\Rightarrow-M=-\left(\frac{1}{9.10}-\frac{1}{8.9}-\frac{1}{7.8}-...-\frac{1}{1.2}\right)\)
\(\Rightarrow-M=\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{7.8}+\frac{1}{8.9}-\frac{1}{9.10}\)
\(\Rightarrow-M=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}-\frac{1}{90}\)
\(\Rightarrow-M=1-\frac{1}{9}-\frac{1}{90}\)
\(\Rightarrow-M=\frac{8}{9}-\frac{1}{90}=\frac{80}{90}-\frac{1}{90}=\frac{79}{90}\)
\(\Rightarrow M=-\frac{79}{90}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(B=\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+....+\frac{1}{9}-\frac{1}{10}\)
\(B=\frac{1}{3}-\frac{1}{10}\)
\(B=\frac{7}{30}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(B=\frac{1}{3.4}-\frac{1}{4.5}-\frac{1}{5.6}-\frac{1}{6.7}-\frac{1}{7.8}-\frac{1}{8.9}-\frac{1}{9.10}\)
\(\Rightarrow B=\frac{1}{3.4}-\left(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}\right)\)
\(\Rightarrow B=\frac{1}{12}-\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\right)\)
\(\Rightarrow B=\frac{1}{3}-\frac{1}{4}-\left(\frac{1}{4}-\frac{1}{10}\right)\)
\(\Rightarrow B=\frac{1}{12}-\frac{6}{40}\)
\(\Rightarrow B=\frac{-1}{15}\)
Ta có: B = 1 9.10 − 1 1.2 + 1 2.3 + ... + 1 7.8 + 1 8.9 = > B = − 79 90