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
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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