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, an = 9 – 5n
and number of terms(n) = 15
So, an = 9 – 5(15) = -66
On putting n = 1, we get a, first term of the A.P.,
a = 9 – 5 = 4
S15 = n[a + an]/2
= 15(4 – 66)/2 = 15 x (-31) = -465
Hence, the sum of the first 15 terms of given A.P. is -465.