\(S=1.2+2.3+3.4+...+98.99\)
\(3S=1.2.3+2.3.3+3.4.3+...+98.99.3\)
\(3S=1.2.3+2.3\left(4-1\right)+3.4\left(5-2\right)+...+98.99\left(100-97\right)\)
\(3S=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+98.99.100-97.98.99\)
\(3S=98.99.100\)
\(S=98.33.100\)
\(S=323400\)