Answer :

superkoopasirf
[tex]S = \frac{1}{2}n(a + l)[/tex]

[tex]a = 11[/tex]
[tex]l = 99[/tex]
[tex]n = 9[/tex]

[tex]S = \frac{1}{2}9(11 + 99)[/tex]
[tex]S = \frac{1}{2}9(110)[/tex]
[tex]S = \frac{990}{2}[/tex]
[tex]S = 495[/tex]
11+22+33+44+55+66+77+88+99
33+77+121+165+99
110+286+99
396+99
495