1/10×11+1/11×12+1/12×13+...1/49×50
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\(\frac{1}{10.11}+\frac{1}{11.12}+\frac{1}{12.13}+.......+\frac{1}{49.50}\)
\(=\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+......+\frac{1}{50}\)
\(=\frac{1}{10}-\frac{1}{50}=\frac{2}{25}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có:C=1/11+12+1/13+...+1/50
=(1/11+1/12+...+1/20)+(1/21+1/22+...+1/30)+(1/31+1/32+...+1/40)+(1/41+1/42+...+1/50)
=(1/20+1/20+...+1/20)+(1/30+1/30+...+1/30)+(1/40+1/40+...+1/40)+(1/50+1/50+...+1/50)
=1/20×10+1/30×10+1/40×10+1/50×10
Nhân với10 vì mỗi tổng đều có10 số hạng
=1/2+1/3+1/4+1/5
=30/60+20/60+15/60+12/60
=77/60
Mik học trên lớp tương tự bài đấy
Hok tốt
![](https://rs.olm.vn/images/avt/0.png?1311)
a, \(\dfrac{10}{17}\) + \(\dfrac{5}{-13}\) - \(\dfrac{11}{25}\) + \(\dfrac{7}{17}\) - \(\dfrac{8}{13}\)
= ( \(\dfrac{10}{17}\) + \(\dfrac{7}{17}\)) - ( \(\dfrac{5}{13}\) + \(\dfrac{8}{13}\)) - \(\dfrac{11}{25}\)
= \(\dfrac{17}{17}\) - \(\dfrac{13}{13}\) - \(\dfrac{11}{25}\)
= 1 - 1 - \(\dfrac{11}{25}\)
= - \(\dfrac{11}{25}\)
b, 0,3 - \(\dfrac{93}{7}\) - 70% - \(\dfrac{4}{7}\)
= 0,3 - 0,7 - ( \(\dfrac{93}{7}+\dfrac{4}{7}\))
= - 0,4 - \(\dfrac{97}{7}\)
= - \(\dfrac{2}{5}\) - \(\dfrac{97}{7}\)
= - \(\dfrac{499}{35}\)
![](https://rs.olm.vn/images/avt/0.png?1311)