A=1.2+2.3+3.4+...+49.50+50.51
3A= 1.2.3+2.3.3+3.4.3+...+49.50.3+50.51.3
3A= 1.2.(3-0)+2.3.(4-1)+3.4.(5-2)+...+49.50.(51-48)+50.51.(52-49)
3A= 0.1.2 - 1.2.3 + 1.2.3- 2.3.4 + 2.3.4 - 3.4.5 + ... + 48.49.50 - 49.50.51 + 49.50.51 - 50.51.52
3A= 50.51.52
3A=132600
A=66300