Ta có: \(\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{2004.2005.2006}\)
\(=\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{2004.2005}-\frac{1}{2005.2006}\)
\(=\frac{1}{1.2}-\frac{1}{2005.2006}\)
\(=\frac{1}{2}-\frac{1}{4022030}\)
\(=-40220295.\)
\(=1-\frac{1}{2}-\frac{1}{3}+\frac{1}{2}-\frac{1}{3}-\frac{1}{4}+...-\frac{1}{2005}+\frac{1}{2004}-\frac{1}{2005}-\frac{1}{2006}\)
\(=1-\frac{1}{2006}=\frac{2005}{2006}\)
sai zùi
=1-1/2-1/3+1/2-1/3-1/4+1/3-1/4-1/5+..........+1/2004-1/2005-1/2006
=>1-1/2006
=>2005/2006
chắc 100%
đằng nào tớ làm đúng trc thì tk mình nhé
