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n/2(2a+(n-1)b)=n/2(7n+11)
(2a+(n-1)b)= (7n+11)
2a+bn-b=7n+11
bn+(2a-b)=7n+11
bn=7n, b=7
2a-b=11
2a-7=11
2a=18, a=9
Un=a+(n-1)b
=9+(n-1)7
Sn = n/2 (a+Un)
n/2 (7n + 11) = n/2 (a+Un)
7n + 11 = 9 +Un
Un = 7n + 2