Question

the sum of the first n terms of an AP is 3n²+4n. Find the 18th term of this AP.
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Answer 1

Expert Answer:

Sn = 3n2 + 4n

First term (a1) = S1 = 3(1)2 + 4(1) = 7

S2 = a1 + a2 = 3(2)2 + 4(2) = 20

a2 = 20 - a1 = 20 - 7 = 13

So, common difference (d) = a2 - a1 = 13 - 7 = 6

Now, an = a + (n - 1)d

a25 = 7 + (25 - 1) 6 = 7 + 24 6 = 7 + 144 = 151

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Answer 2

Step-by-step explanation:

✏️Given Sn = 3n2  + 4n S1 = 3×1 + 4×1 = 7. 

S2 = 3×4 + 4×2 = 12 + 8 = 20.

 Therfore First term a1 = 7

&  Second term a2 = 20 - 7 = 13

and d = 13 - 7 = 6.

18th term=7+(18-1)6

A18=7+17×6

A18=109

Therefore,18th term=109...

✒️ Hope it helps ☺️✌️♥️...