3s1=(2.3+3.4+4.5+5.6+...+15.16+16.17).3
3s1=2.3.3+3.4.3+4.5.3+5.6.3+....+15.16.3+16.17.3
3s1=2.3(4-1)+3.4(5-2)+4.5(6-3)+5.6(7-4)+...+15.16(17-14)+16.17(18-15)
3s1=2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+...+15.16.17-14.15.16+16.17.18-15.16.17
3s1=(16.17.18-1.2.3)/3
s1=1630
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