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2015-11-09T19:01:59+05:30

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Sum of first n terms of the AP = n² + 8n
Sum of first 1 term = first term, a = 1² + 8×1 = 9
Sum of first 12 terms = 12² + 8×12 = 240

But we also know that sum of first n terms = (n/2)[a + an]
(an = nth term)

So sum of first 12 terms = (12/2)[9 + a₁₂]
⇒ 240 = 6×(9 + a₁₂)
⇒ 9 + a₁₂ = 240/6 = 40
⇒ a₁₂ = 40- 9 
⇒ a₁₂ = 31

12th term is 31.
2 5 2
but ansewr is 130
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