How many terms of Ap -15,-13,-11 are needed to make the sum -55 ? Explain the reason for double answer reason foe double answer..plz

Dear student
The given AP is-15,-13,-11,...Here the first term,a=-15 and common difference,d=second term-first term=-13--15=-13+15=2Also, given  sum of n terms=-55i.e. Sn=-55
Hence sum of 5 terms and 11 terms of the given AP is -55Note: Here we had a quadratic equation in n, so we will get 2 values of nand hence sum of 5 terms and 11 terms of the given AP is -55

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Reason for double answer ....plz i know how to solve this question but dont know the reason for double answer.....plz the reason
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ap = -15 ,-13, -11 ....
sn= -55
plz give full ap.
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