A=1.2 +2.3+3.4+...+98.99
3A=1.2.3+2.3.3+3.4.3+...+98.99.3
3A=1.2.3+2.3.(4-1)+3.4.(5-2)+...+98.99.(100-97)
3A=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+98.99.100-97.98.99
3A=(1.2.3-1.2.3+2.3.4-2.3.4+...+97.98.99-97.98.99)+98.99.100
3A=98.99.100
A= $\frac{98.99.100}{3}$
A= 323400