Question

find the sum of integers which are divisible by 2 and 3 from 1 to 50

Answer 1

AP for such numbers is 6,12,18,.......48

Here, a = 6
  and, d = 6

Let there be n terms in this AP

so, an = 48
    a + (n-1)d = 48
          (n-1)6 = 42
         so, n-1 = 7
        so, n = 8

So, S = S8 = 8/2 [a+l]
                  = 4[a+l]
                  = 4[6+48]
                  = 54*4
                  = 216