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]`

S₇ = `7/2[2(18) + (7 - 1)8]`

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

= 7(18 + 3 × 18)

= 7(18 + 54)

= 7 × 72

= 504