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Q.16 - Mathematics

Sum

Q.16

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Solution

`"Sum of first 15 term of an A.P.=0"`

`⇒ S_15=0`

`⇒15/2[2a+14]=0`

`⇒ 2a+14d=0`

`⇒a+7d=0.................(1)`

`4^(th) "term" =t_4=12`'

⇒a+3d=12 ..............(2)

Subtracting (2) from (1), we get 

`4d=-12⇒d=3`

`⇒ a+7xx(-3)=0`

`⇒a=21`

`∴ 12^(th) "term" = t_12=21+11xx(-3)=21-33=-12`

Concept: Arithmetic Progression - Finding Sum of Their First ‘N’ Terms.
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