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
Given Sn = 3n2 + 4n,
On putting n = 1, we get the first term(a), S1 = a = 3(1)2 + 4(1) = 7
On putting n = 2 gives S2 = a + a + d = 3(2)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.