Question

Find the sum of first seven numbers which are multiples of 2 as well as of 9.

Answer 1

For finding the sum of first seven numbers which are multiples of 2 as well as of 9.

Take LCM of 2 and 9 which is 18.

So, the series becomes 18, 36, 54 ….

Here, first term (a) = 18,

Common difference d) = 36 – 18 = 18

∵ `S_n = n/2 [2a + (n - 1)d]`

`S_7 = 7/2 [2(18) + (7 - 1)8]`

= `7/2[36 + (6 xx 18)]`

= `7(18 + 3 xx 18)`

= `7(18 + 54)`

= `7 xx 72`

= 504