y = 14 x 36 : 63 = 8

x = 14 x 4 : 8 = 7

\(\frac{x}{6}+\frac{x}{10}+\frac{x}{15}+\frac{x}{21}+\frac{x}{28}+\frac{x}{36}+\frac{x}{45}+\frac{x}{55}+\frac{x}{66}+\frac{x}{78}+\frac{x}{78}=\frac{220}{39}\)

\(\Leftrightarrow x\left(\frac{1}{6}+\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+\frac{1}{28}+\frac{1}{36}+\frac{1}{45}+\frac{1}{55}+\frac{1}{66}+\frac{1}{78}+\frac{1}{78}\right)=\frac{220}{39}\)

\(\Leftrightarrow x\cdot\frac{20}{39}=\frac{220}{39}\Rightarrow x=11\)

\(\frac{x}{6}+\frac{x}{10}+\frac{x}{15}+\frac{x}{21}+\frac{x}{28}+\frac{x}{36}+\frac{x}{45}+\frac{x}{55}+\frac{x}{66}+\frac{x}{78}+\frac{x}{78}=\frac{220}{39}\)

\(=>x=\frac{1}{6}+\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+\frac{1}{28}+\frac{1}{36}+\frac{1}{45}+\frac{1}{55}+\frac{1}{66}+\frac{1}{78}+\frac{1}{78}=\frac{220}{39}\)

\(x\cdot\frac{20}{39}=\frac{220}{39}\)

\(x=\frac{220}{39}:\frac{20}{39}=11\)

\(\Leftrightarrow\frac{1}{2}+\left(\frac{2}{56}+\frac{2}{72}+...+\frac{2}{x\left(x+1\right)}\right)=\frac{3}{10}\)

\(\Leftrightarrow\frac{1}{2}+2.\left(\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+...+\frac{1}{x}-\frac{1}{x+1}\right)=\frac{3}{10}\)

\(\Leftrightarrow2.\left(\frac{1}{7}-\frac{1}{x+1}\right)=\frac{3}{10}-\frac{1}{2}=-\frac{1}{5}\)

\(\Leftrightarrow\frac{1}{7}-\frac{1}{x+1}=-\frac{1}{5}:2=-\frac{1}{10}\)

\(\Leftrightarrow\frac{1}{x+1}=\frac{1}{7}-\left(-\frac{1}{10}\right)=\frac{17}{70}\)

\(\Rightarrow17x+17=70\)

=> không tồn tại n vì n là số tự nhiên

=2/42+2/56+....+2/x.(x+1)

=2/6.7+2/7.8+.....+2/x.(x+1)

=2.(1/6.7-1/7.8+....+1/x.(x+1)

=2.(1/6-1/7+1/7-1/8+...+1/x-1/x+1)

=2.(1/6-1/x+1)

=2.(x-5/6x+6)

=2x-10/6x+6

bn vt thieu de nha = bao nhieu bn tu tinh 

nhân 2 cả tử và mẫu lên=> ta có 2/6.7 + 2/7.8........

tự làm nốt nhé ;-)

1/21 + 1/28 + 1/36 + ...+ 1/x(x+1)

=> 2/42 + 2/56 + 2/72 +....+ 2/x(x+1)

=> 2.(1/42 + 1/56 + 1/72 + ... + 1/x.(x+1))

=> 2 .(1/6.7 + 1/7.8 + 1/8.9 + ..+ 1/x.(x+1))

=> 2. ( 1/6 - 1/7 + 1/7-1/8 + ...+ 1/x - 1/x+1

=> 2 . (1/6 - 1/x+1)

=>1/3 - 2/x+1