Question

Find the sum of all natural numbers between 100 and 1000 which are multiple of 7.OR21

Answer 1

$\huge\bold\star\red{QUESTION}\star$

Find the sum of all natural numbers between 100 and 1000 which are multiple of 7.OR21

$\huge\bold\star\red{ANSWER}\star$

Natural numbers b/w 100 and 1000 which are multiples of 5 are :

105,110,115,.........,990,995.

a=105, d=5

an=995

a+(n−1)d=995

105+(n−1)5=995

(n−1)5=890

n−1=178

n=179

Hence,

Sn=2n[2a+(n−1)d]

S179=2179[210+890]

S179=2179×1100=98450