Question

Find the sum of first $40$ positive integers divisible by $7$.

Answer 1

First term $=a=7$

Difference between terms$=d=7$

Number of terms $=n=40$

Sum$=S=\frac{n}{2}(2a+(n-1\left)d\right))$

$S=\frac{40}{2}(2\times 7+(40-1\left)7\right))$

$S=20\times (2\times 7+(39\left)7\right))$

$S=20\times (14+273)$

$S=20\times 287$

$S=5740$