Question

find the sum of all natural numbers between 100 and 1000 which multipies of 5

Answer 1

The AP is
100,105,115.........1000
Here a=100, d=5, l=1000
Let the no. of terms in the AP be n
Therefore,
tn=l
a+(n-1)d=1000
100+(n-1)5=1000
100+5n-5=1000
95+5n=1000
5n=905
n=905/5
n=181
Sn=n/2(a+l)
S181=181/2(100+1000)
=181/2(1100)
=181×550
=99550