\(A=\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{2015.2016.2017}\)
\(\Rightarrow2A=\frac{2}{1.2.3}+\frac{2}{2.3.4}+...+\frac{2}{2015.2016.2017}\)
\(=\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...-\frac{1}{2016.2017}\)
\(=\frac{1}{2}-\frac{1}{2016.2017}< \frac{1}{2}\)
\(\Rightarrow2A< \frac{1}{2}\Rightarrow A< \frac{1}{4}\)