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 an arithmetic progression whose nth term is given by yn = 9 – 5n . And we have to find out sum of this progression of first 15 terms.

CBSE DHANPAT RAI publication

CBSE Mathematics Class 10th

Question 5(iv)

Given A.P. has nth term, a

_{n}= 9 – 5nand number of terms(n) = 15

So, a

_{n}= 9 – 5(15) = -66On putting n = 1, we get a, first term of the A.P.,

a = 9 – 5 = 4

S

_{15}= n[a + a_{n}]/2= 15(4 – 66)/2 = 15 x (-31) = -465

Hence, the sum of the first 15 terms of given A.P. is -465.