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 asked to find the sum of 15 multiples of 8.
CBSE DHANPAT RAI publication
CBSE Mathematics Class 10th
Question 12(i)
First 15 multiples of 8 are 8, 16, 24, …… , 120.
These multiples form an A.P. with first term(a) = 8,
Common difference(d) = 16 − 8 = 8
and number of terms(n) = 15.
We know sum of n terms of an A.P. is given by, Sn = n[2a + (n − 1)d] / 2.
S15 = 15[2(8) + (15 − 1)8]/2
= 15[16 + 112]/2
= 15[64] = 960
Hence, the sum of the first 15 multiples of 8 is 960.