1/30+1/42+1/56+1/72+....+1/210
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A= 1/30 + 1/42+1/56+1/72+....+1/210
A= 1/5.6+1/6.7+1/7.8+1/8.9+....+1/14.15
A= 1/5 - 1/6+1/6-1/7+1/7-1/8+1/8-1/9+.....+1/14-1/15
A= 1/5 - 1/15
A = 2/15
![](https://rs.olm.vn/images/avt/0.png?1311)
A= 1/30 +1/42+1/56+1/72+....+1/210
A=1/5x6 +1/6x7+1/7x8+1/8x9+...+1/14x15
A=1/5 -1/6+1/6-1/7+1/7-1/8+1/8-1/9+.....+1/14-1/15
A= 1/5 - 1/15
A= 2/15
\(\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+...+\frac{1}{210}\)
=\(\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+...+\frac{1}{14.15}\)
\(=\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+...+\frac{1}{14}-\frac{1}{15}\)=\(\frac{1}{5}-\frac{1}{15}\)
=\(\frac{3}{15}-\frac{1}{15}\)
=\(\frac{2}{15}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(A=\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+...+\frac{1}{210}\)
\(A=\frac{1}{5\cdot6}+\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+\frac{1}{8\cdot9}+...+\frac{1}{14\cdot15}\)
\(A=\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{14}-\frac{1}{15}\)
\(A=\frac{1}{5}-\frac{1}{15}=\frac{2}{15}\)
\(A=\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+...+\frac{1}{210}\)
\(A=\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+...+\frac{1}{14.15}\)
\(A=\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+...+\frac{1}{14}-\frac{1}{15}\)
\(A=\frac{1}{5}-\frac{1}{15}\)
\(A=\frac{2}{15}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
= 1/ 5.6 + 1/ 6.7 + 1/7.8 + 1/8.9 +.....+ 1/ 14.15
= 1/ 5 - 1/6 + 1/6 - 1/7 +......+1/14 - 1/15
= 1/5 - 1/ 15
= 3/15 - 1/15 = 2/ 15
![](https://rs.olm.vn/images/avt/0.png?1311)
\(A=\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{14.15}\)
\(A=\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{14}-\frac{1}{15}\)
\(A=\frac{1}{5}-\frac{1}{15}\)
\(A=\frac{2}{15}\)
\(A=\frac{1}{30}+\frac{1}{40}+\frac{1}{56}+\frac{1}{72}+...+\frac{1}{210}\)
\(A=\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+...+\frac{1}{14.15}\)
\(A=\frac{6-5}{5.6}+\frac{7-6}{6.7}+\frac{8-7}{7.8}+\frac{9-8}{8.9}+...+\frac{15-14}{14.15}\)
\(A=1-\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+...+\frac{1}{14}-\frac{1}{15}\)
\(A=1-\frac{1}{15}\)
\(A=\frac{14}{15}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(A=\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+....+\frac{1}{14.15}\)
\(=\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+.....+\frac{1}{14}-\frac{1}{15}\)
\(=\frac{1}{5}-\frac{1}{15}\)
\(=\frac{2}{15}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(A=\frac{1}{30}+\frac{1}{42}+...+\frac{1}{210}\)
\(A=\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{14.15}\)
\(A=\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{14}-\frac{1}{15}\)
\(A=\frac{1}{5}-\frac{1}{15}\)
Tự tính nha :)
\(B=\frac{2}{2.3}+\frac{2}{3.4}+...+\frac{2}{99.100}\)
\(B=2\left(\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{99.100}\right)\)
\(B=2\left(\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{99.100}\right)\)
\(B=2\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{99}-\frac{1}{100}\right)\)
\(B=2\left(\frac{1}{2}-\frac{1}{100}\right)\)
Tự làm
![](https://rs.olm.vn/images/avt/0.png?1311)
1/5.6 + 1/6.7 + 1/7.8 + 1/8.9 . ..... . 1/14.15
= 1/5 + 1/6 - 1/6 + 1/7 + 1/7 - 1/8 + 1/9 - ..... + 1/14 - 1/15
= 1/5 - 1/15
= 2/15
1/30+1/42+1/56+1/72+....+1/210
=1/5.6+1/6.7+1/7.8+1/8.9+...+1/14.15
=1/5-1/6+1/6-1/7+1/7-1/8+....+1/14-1/15
=1/5-1/15
=2/15