Question

Question 31
Find the sum of first seven numbers which are multiples of 2 as well as of 9.

Answer 1

To find 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 numbers are 18, 36, 54, ....
Here, first term, a = 18, common difference, d = 36 – 18 = 18
$S_n=\frac{n}{2}[2a+(n-1\left)d\right],S_7=\frac{7}{2}\left[2\right(18)+(7-1\left)18\right]$
$=\frac{7}{2}[36+6\times 18]=7(18+3\times 18)$
$=7(18+54)=7\times 72=504$