What is the sum of the first 15 terms of the se... - SS2 Mathematics Sequence and Series Question
What is the sum of the first \(15\) terms of the sequence \(- 9,\ - 6,\ - 3,\ \ldots\)
A
450
B
- 450
C
- 180
D
180
correct option: d
\[S_{n} = \frac{n}{2}\lbrack 2a + (n - 1)d\rbrack\]
\[ {S_{15} = \frac{15}{2}\lbrack 2( - 9) + (15 - 1)(3)\rbrack}\]
\[S_{15} = \frac{15}{2}\lbrack - 18 + (14)(3)\rbrack\]
\(S_{15} = \frac{15}{2}\lbrack - 18 + 42\)]
\[S_{15} = \frac{15}{2}\lbrack 24\rbrack\]
\(S_{15} = 180\)
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