NCERT Solutions
Class 11 Maths
Sequences and Series

Ex.Misc.Q.5
Find the sum of integers from 1 to 100 that are divisible by 2 or 5.
The integers from 1 to 100, which are divisible by 2, are 2, 4, 6… 100.
This forms an A.P. with both the first term and common difference equal to 2.
100 = 2 + (n –1)2
50 = 1 + n –1
n = 50
So, 2 + 4 + 6 + ……100 = (50 ÷ 2) [2 × 2 + (50 - 1)2]
= 25[4 + 98]
= 25 × 102
= 2550
The integers from 1 to 100, which are divisible by 5, are 5, 10… 100.
This forms an A.P. with both the first term and common difference equal to 5.
So, 100 = 5 + (n –1) 5
5n = 100
n = 20
So, 5 + 10 + 15 + ……100 = (20 ÷ 2) [2 × 5 + (20 - 1)5]
= 10[10 + 95]
= 10 × 105
= 1050
The integers, which are divisible by both 2 and 5, are 10, 20, … 100.
This also forms an A.P. with both the first term and common difference equal to 10.
So, 100 = 10 + (n –1) (10)
100 = 10n
n = 10
So, 10 + 20 + 30 + ……100 = (10 ÷ 2) [2 × 10 + (10 - 1)10]
= 5[20 + 90]
= 5 × 110
= 550
Hence, required sum = 2550 + 1050 – 550 = 3050
Thus, the sum of the integers from 1 to 100, which are divisible by 2 or 5 is 3050.