Question

Find the sum fo first forty positive integers divisible by 6.

Answer 1

The positive integers divisible by 6 are 6,12,18,....... up to 40 terms

the given series is in arthimetic progression with first term a=6 and common difference d=6

the sum of n terms of an A.p is

Sum of n terms, $S_n=\frac{n}{2}[2a+(n-1\left)d\right]$

$=\frac{40}{2}\times 2\left(6\right)+\left(39\right)6$

$=2012+234=20\times 246=4920$