\(S=\frac{2}{1.2.3}+\frac{2}{2.3.4}+...+\frac{2}{2009.2010.2011}\)
\(S=2.\left(\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{2009.2010.2011}\right)\)
\(S=2.\frac{1}{2}.\left(\frac{1}{1.2}-\frac{1}{2.3}+...+\frac{1}{2009.2010}-\frac{1}{2010.2011}\right)\)
\(S=1.\left(\frac{1}{1.2}-\frac{1}{2010.2011}\right)\)
\(S=\frac{1}{1.2}-\frac{1}{2010.2011}\)
\(S=\frac{1}{2}-\frac{1}{2010.2011}< \frac{1}{2}\)
Vậy \(S< \frac{1}{2}\)
Chúc bạn học tốt !!!
Áp dụng công thức :
\(\frac{1}{2}.\left(\frac{1}{1.2}-\frac{1}{2.3}\right)=\frac{1}{2}.\left(\frac{1}{2}-\frac{1}{6}\right)=\frac{1}{2}.\frac{2}{6}=\frac{1}{6}=\frac{1}{1.2.3}\)
Chúc bạn học tốt !!!