Question

4. Find the sum of all integers between 100 and 1000 which are divisible by 9.

Answer 1

Find the first number after 100 which is divisible by 9 i.e. 108

a=108

Last number which is divisible by 9 is

999

So number of terms -

(999-108)+1

= 892

In an AP

a=108

d=9

n=892

Formula for sum of terms

Sn=n/2(2a+(n-1)d)

892/2(2*108+891*9)

446(216+8019)

446(8235)

3672810