Question

Find the sum of all natural numbers between 200 and 1502 which are exactly divisible by 3.

Answer 1

ap is 201, 204,........1500
a = 201
d = 3
an = 1500
an = a + (n-1)d
1500 = 201 + (n-1)3
1299 = (n-1)3
433 = n - 1
therefore, n =434
Sn=n/2x(a first+a last)
=434/2x(201+1502)
=217x1701
=369117