This is the basic and conceptual question from Chapter name- Arithmetic Progression

Chapter number- 9

Exercise 9.6

In this question we have been given an arithmetic progression having nth term as xn = 6 – n

And we have to find its sum of first 15 terms.

CBSE DHANPAT RAI publications

Class:- 10th

Solutions of CBSE Mathematics

Question 5(iii)

Given A.P. has nth term, a

_{n}= 6 – nand number of terms(n) = 15

So, a

_{n}= 6 – n = -9On putting n = 1, we get a, first term of the A.P.,

a = 6 – 1 = 5

S

_{15}= n[a + a_{n}]/2= 15(5 – 9)/2 = 15 x (-2) = -30

Hence, the sum of the first 15 terms of given A.P. is -30.