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

sum=569117

The ap is 201, 204,........1500
a = 201    d = 3   a_n = 1500
a_n = a + (n-1)d
1500 = 201 + (n-1)3
1299 = (n-1)3
433 = n - 1
therefore, n = 434

S_n = n/2 ( a + a_n )
  = 434/2 (201+1500)
  = 217 * 1701
 = 369117

the sum of all natural no.s between 200 and 1502 is 369117