This is an arithmetic progression based question from Chapter name- Arithmetic Progression
Chapter number- 9
Exercise – 9.6
In this question we have been given the arithmetic progression whose nth term is given by bn=5+2n . Also we have to find the sum of first 15 terms of it.
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Understanding CBSE Mathematics
Class :- 10th
Question no 5(ii)
Given A.P. has nth term, an = 5 + 2n and
number of terms(n) = 15
So, an = 5 + 2(15) = 35
On putting n = 1, we get a, first term of the A.P.,
a = 5 + 2(1) = 7
S15 = n[a + an]/2
= 15(7 + 35)/2 = 15 x 21 = 315
Hence, the sum of the first 15 terms of given A.P. is 315.