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Question 41. The sum of first n terms of an A.P. is 3n2 + 4n. find the 25th term of this A.P.

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One of the most important and exam oriented question from Chapter name- Arithmetic Progression
Class 10th
Chapter number- 9
Exercise :- 9.6
This type of question has been asked in previous years exams.

In this question we have been given that The sum of first n terms of an A.P. is 3n2 + 4n.

Now we have to find the 25th term of this A.P.

CBSE DHANPAT RAI publication
CBSE Mathematics Class 10th
Question 41

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1 Answer

  1. Given Sn = 3n2 + 4n,

    On putting n = 1, we get the first term(a), S1 = a = 3(1)+ 4(1) = 7

    On putting n = 2 gives S2 = a + a + d = 3(2)+ 4(2) = 20

    => d = 20 – 2a

     

     

    => d = 20–14 = 6

    The 25th term of the A.P., a25 = a + (25 – 1)d

    = 7 + (24)8

    = 7 + 144

    = 151

    Hence, the 25th term of the A.P. is 151.

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