\(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{x.\left(x+1\right)}=\frac{996}{997}\)
\(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{x}-\frac{1}{x+1}=\frac{996}{997}\)
\(1-\frac{1}{x+1}=\frac{996}{997}\)
\(\frac{1}{x+1}=1-\frac{996}{997}\)
\(\frac{1}{x+1}=\frac{1}{997}\)
\(\Rightarrow x+1=997\)
\(x=997-1\)
\(x=996\)