# Plzzz answer ----------------------- (1).The sum of first n terms of an AP is given by n (square)+8n.find its 12th term

2
by anujasanthosh14

2015-11-09T19:15:41+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.
2015-11-09T19:17:58+05:30

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Certified answers contain reliable, trustworthy information vouched for by a hand-picked team of experts. Brainly has millions of high quality answers, all of them carefully moderated by our most trusted community members, but certified answers are the finest of the finest.
Since ,

Sn = n² + 8n

Putting n = 1

S1 = 1 + 8 = 9

so first term is 9 i.e. a

Now putting n = 2

S2 = 4 + 8×2
S2 = 4 + 16 = 20

Now , s2-s1 = a2

a2= 20 - 9 = 11
so , d is a2-a1 = 2
so 12th term will be :-

a12 = a + 11d

a12 = 9 + 11×2

a12 = 9 + 22 = 31
hmm... my teacher told that s2-s1 = d
Anyway, it's upto you... I am not going to argue. I gave a solution, it's upto you what you want to do.
ohkk.. bhaiya m confirm krta hu :) ,
umm.. u r ryt bhaiya s2-s1 = a2
(a1+a2) - a1 = a2