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CBSE Class 10 Answered

The sum of first n terms of an AP is 3n2 + 4n. Find the 25th term of this AP.
Asked by Topperlearning User | 22 Sep, 2017, 09:24: AM
answered-by-expert 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

Answered by | 22 Sep, 2017, 11:24: AM

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