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Sagot :
Given:
In an arithmetic sequence,
[tex]a_1=2[/tex]
[tex]a_i=a_{i-1}-3[/tex]
To find:
The sum of the first 335 terms in the given sequence.
Solution:
The recursive formula of an arithmetic sequence is:
[tex]a_i=a_{i-1}+d[/tex] ...(i)
Where, d is the common difference.
We have,
[tex]a_i=a_{i-1}-3[/tex] ...(ii)
On comparing (i) and (ii), we get
[tex]d=-3[/tex]
The sum of first i terms of an arithmetic sequence is:
[tex]S_i=\dfrac{i}{2}[2a+(i-1)d][/tex]
Putting [tex]i=335,a=2,d=-3[/tex], we get
[tex]S_{335}=\dfrac{335}{2}[2(2)+(335-1)(-3)][/tex]
[tex]S_{335}=\dfrac{335}{2}[4+(334)(-3)][/tex]
[tex]S_{335}=\dfrac{335}{2}[4-1002][/tex]
[tex]S_{335}=\dfrac{335}{2}(-998)[/tex]
On further simplification, we get
[tex]S_{335}=335\times (-499)[/tex]
[tex]S_{335}=-167165[/tex]
Therefore, the sum of the first 335 terms in the given sequence is -167165.
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