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r=7
n=100
an=a1+(n-1)*r
a100=7k+2+99*7
a100=7k+695
Sn=(a1+an)*n/2
(7k+2+7k+695)*100 =43950 /:50
2*7k+697 =879
2*7k=879-697
7k=182:2
7k=91
a1=91+2 =93
a100=91+695 =768