Find the sum of all natural numbers amongst first one thousand numbers which are neither divisible 2 or by 5
Ans: Sum of all natural numbers in first 1000 integers which are not divisible by 2 i.e. sum of odd integers.
1 + 3 + 5 + .......... + 999
n = 500
S_{500} = 500/2 [1 + 999]
= 2,50,000
No's which are divisible by 5
5 + 15 + 25 ........ + 995
n = 100
Sn = 100/2 [5 + 995]
= 50 x 1000 = 50000
∴ Required sum = 250000 - 50,000
= 200000