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Sagot :
Given:
The terms of a sequence are:
[tex]-6,-\dfrac{11}{2},-5[/tex]
To find:
The number of terms whose sum is -25.
Solution:
We have, the given sequence:
[tex]-6,-\dfrac{11}{2},-5[/tex]
Here, the first term is -6.
[tex]-\dfrac{11}{2}-(-6)=-5.5+6[/tex]
[tex]-\dfrac{11}{2}-(-6)=0.5[/tex]
Similarly,
[tex]-5-(-\dfrac{11}{2})=-5+5.5[/tex]
[tex]-5-(-\dfrac{11}{2})=0.5[/tex]
The difference between consecutive terms are same. So, the given sequence is an arithmetic sequence with common difference 0.5.
The sum of n terms of an arithmetic sequence is:
[tex]S_n=\dfrac{n}{2}[2a+(n-1)d][/tex]
Where, a is the first term, d is the common difference.
Putting [tex]S_n=-25,a=-6,d=0.5[/tex], we get
[tex]-25=\dfrac{n}{2}[2(-6)+(n-1)0.5][/tex]
[tex]-50=n[-12+0.5n-0.5][/tex]
[tex]-50=-12.5n+0.5n^2[/tex]
[tex]0=0.5n^2-12.5n+50[/tex]
Splitting the middle term, we get
[tex]0.5n^2-2.5n-10n+50=0[/tex]
[tex]0.5n(n-5)-10(n-5)=0[/tex]
[tex](0.5n-10)(n-5)=0[/tex]
Using zero product property, we get
[tex](0.5n-10)=0[/tex] and [tex](n-5)=0[/tex]
[tex]n=\dfrac{10}{0.5}[/tex] and [tex]n=5[/tex]
[tex]n=20[/tex] and [tex]n=5[/tex]
Therefore, the sum of either 5 or 20 terms is -25.
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