Đặt  \(A=\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+\frac{1}{28}\)

\(A=\frac{2}{6}+\frac{2}{12}+\frac{2}{20}+\frac{2}{30}+\frac{2}{42}+\frac{2}{56}\)

\(A=\frac{2}{2.3}+\frac{2}{3.4}+\frac{2}{4.5}+\frac{2}{5.6}+\frac{2}{6.7}+\frac{2}{7.8}\)

\(A=2\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+....+\frac{1}{7}-\frac{1}{8}\right)\)

\(A=2\left(\frac{1}{2}-\frac{1}{8}\right)\)

\(\Rightarrow A=2\cdot\frac{3}{8}=\frac{3}{4}\)

\(=1+\frac{1}{1.3}+\frac{1}{3.2}+\frac{1}{2.5}+\frac{1}{5.3}+\frac{1}{3.7}+\frac{1}{7.4}+\frac{1}{4.9}+\frac{1}{9.5}\)

\(=1+1-\frac{1}{5}\)

\(=\frac{10}{5}-\frac{1}{5}\)

\(=\frac{9}{5}\)

Ai thấy đúng thì

ra 9/5 bạn nhé

k cho mk mk kb 

\(\frac{9}{5}nha\)

p=1-1/2.5-1/3.5-1/1.3-1/4.7-1/2.3-1/3.7

p=1-(1/2.1/5-1/3.1/5)-(1/1.1/3-1/2.1/3)-(1/4.1/7-1/3.1/7)

p=1-(1/5.(1/2-1/3))-(1/3.(1-1/2))-(1/7.(1/4-1/3)

p=1-(1/5.1/6)-(1/3.1/2)-(1/7.-1/12)

p=1-1/30-1/6+1/84

p=341/420

=1/4 nha bạn.

P=1-1/3-1/6-1/10-1/15-1/21-1/28

P=2(1/2-1/6-1/12-1/20-1/30-1/42-1/56)

P=2(1/1.2-1/2.3-1/3.4-1/4.5-1/5.6-1/6.7-1/7.8)

P=2(1-1/2-1/2+1/3-1/3+1/4-1/4+1/5-1/5+1/6-1/6+1/7-1/7+1/8)

P=2.1/8

P=1/4

\(\frac{9}{11}\)nha kb đi

lấy (1/3 + 1/15 +1/10 + 1/21 ) + (1/36 + 1/28 + 1/6) + (1/45 + 1/55)

      =  (4/50 + 3/70) + 2/100

      = 7/120 + 2/100

      = 9/220

= 9/11