Verified Answer :

\(S = \frac{1}{2}n(a + l)\)

\(a = 11\)
\(l = 99\)
\(n = 9\)

\(S = \frac{1}{2}9(11 + 99)\)
\(S = \frac{1}{2}9(110)\)
\(S = \frac{990}{2}\)
\(S = 495\)

11+22+33+44+55+66+77+88+99
33+77+121+165+99
110+286+99
396+99
495