Could someone explain this please? Thank you.

Express the series 11+18+27+...171 using sigma notation.

a1+a2+a3+...ak
11+18+27+...171

S = 11 + 18 + 27 + 38 + 51 + 65 + 82 + .........+Tn

S = 11 + 18 + 27 + 38 + 51 + 65 + ................ + Tn
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0 = 11 + (7 + 9 + 11 + 13 + ....(n-1).terms.................) - Tn
0 = 11 + (n - 1)/2 [14 + 2(n - 2)] = 11 + (n - 1)(n + 5) - Tn
Tn = n^2 + 4n + 6. The given series in sigma notation is as follows.
SIGMA[k^2 + 4k + 6] the lower limit being k = 1 and the upper limit k = n