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Answered

Find the sum of the first 15 terms of the arithmetic sequence 17, 12, 7, 2
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Sagot :

CHACHASMOOTHIN

Answer:

[tex]S_{n}=-270[/tex]

Step-by-step explanation:

first find [tex]a_{15}[/tex]:

[tex]a_{n}=a_{1}+(n-1)d\\a_{15}=17+(15-1)(-5)\\a_{15}=17+(14)(-5)\\a_{15}=17+(-70)\\a_{15}=-53[/tex]

Solution:

[tex]S_{n} =\frac{n}{2} (a_{1}+a_{n})\\S_{n}=\frac{15}{2} [17+(-53)]\\S_{n}=\frac{15}{2} (-36)\\S_{n}=7.5 (-36)\\S_{n}=-270[/tex]

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