Find the sum of first seven numbers
Question:

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

Solution:

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 (of) = 36 -18 = 18

$\therefore \quad S_{7}=\frac{n}{2}[2 a+(n-1) d]=\frac{7}{2}[2(18)+(7-1) 18]$

$=\frac{7}{2}[36+6 \times 18]=7(18+3 \times 18)$

$=7(18+54)=7 \times 72=504$