= 12009 nha mình đúng đấy mình nha 

thanks

Kết quả :

1/3 + 1/6 + 1/10 + 1/15 + ...+2/X x (X + 1 )

= 2 x ( 1/6 + 1/12 +1/20 + 1/30 + ...+ 1/X  x ( X + 1 )

= 2 x  (1/ 2x3 + 1/3x4 + 1/4x5 + 1/5x6 +...+1/x - 1/x+1 )

= 2 x  ( 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 -1/6 + ...+1/x - 1/x+1 )

= 2 x  ( 1/2 - 1/x+1 ) = 2007/2009

=   1 - 2/x+ 1 = 2007/2009

           2/x+1 = 1 - 2007/2009

           2/x + 1= 2/2009

              x + 1 = 2009

                    x = 2009 - 1

                    x  = 2008

THANKS BẠN NHA

nhat beo oi

\(\frac{2}{6}+\frac{2}{12}+...+\frac{2}{x\left(x-1\right)}=\frac{2007}{2009}\)

\(2\cdot\left(\frac{1}{6}+\frac{1}{12}+..+\frac{1}{x\left(x-1\right)}\right)=\frac{2007}{2009}\)

\(\frac{1}{6}+\frac{1}{12}+..+\frac{1}{x\left(x-1\right)}=\frac{2007}{2009}:2\)

\(\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{\left(x-1\right)x}=\frac{2007}{4018}\)

\(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+..+\frac{1}{x-1}-\frac{1}{x}=\frac{2007}{4018}\)

\(\frac{1}{2}-\frac{1}{x}=\frac{2007}{4018}\)

\(\frac{1}{x}=\frac{1}{2}-\frac{2007}{4018}\)

\(\frac{1}{x}=\frac{1}{2009}\)

=> x = 2009 

\(\frac{1}{2.3}=\frac{1}{2}-\frac{1}{3}\) trong đó 3 = x ; 2 = x - 1 

\(\frac{1}{\left(x-1\right)x}=\frac{1}{x-1}-\frac{1}{x}\)

ĐẶt A = \(\frac{1}{3}+\frac{1}{6}+...+\frac{2}{x\left(x-1\right)}-\frac{2007}{2009}\)

      A = \(\frac{2}{6}+\frac{2}{12}+\frac{2}{30}\cdot\cdot\cdot+\frac{2}{x\left(x-1\right)}-\frac{2007}{2009}\)

      A = \(\frac{2}{2.3}+\frac{2}{3.4}+\frac{2}{4.5}+..+\frac{2}{\left(x-1\right)x}-\frac{2007}{2009}\)

      A = \(\frac{1}{2}-\frac{1}{3}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{x-1}-\frac{1}{x}-\frac{2007}{2009}\)

A     \(=\frac{1}{2}-\frac{1}{x}-\frac{2007}{2009}\)