Question

find the sum of all number between 200 and 400 which are divided by 3​

Answer 1

Answer

The numbers lying between 200 and 400 which are divisible by 7 are 203,210,217,...399

∴ First term, a=203

Last term, a

n

=399 & Common difference, d=7

Let the number of terms of the A.P. be n.

∴a

n

=399=a+(n−1)d

⇒399=203+(n−1)7

⇒7(n−1)=196

⇒n−1=28⇒n=29

∴S

29

=

2

29

(203+399)

=

2

29

(602)=(29)(301)=8729

Thus, the required sum is 8729