NCERT Class X Chapter 5: Arithmetic Progression Exercise 5.3 Question 13

NCERT Class X Chapter 5: Arithmetic Progression Exercise 5.3 Question 13

Question:

Find the sum of the first 15 multiples of 8.

Given:

First 15 multiples of 8 are required.

To Find:

The sum of the first 15 multiples of 8.

Formula:

The sum of an arithmetic series is given by: Sn = n 2 (a1 + an), where 

n is the number of terms, 

a1 is the first term, and 

an is the last term.

Solution:

First 15 multiples of 8 are required are 8, 16, 24, .... , 120

This forms an AP, with  

first term a1 = 8, 

common difference d = 8 and 

last term an = 120

The sum of an arithmetic series till 15 is given by: S15 = 15 2 ( 8 + 120 ), where

⇒ S15 = 15 × 128 2

⇒ S15 = 15 × 64

⇒ S15 = 960

Result:

The sum of the first 15 multiples of 8 is 960.

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