Question

Find the sum of even numbers between 10 to 40

Answer 1

sum of all no. till x =
$\frac{x(x + 1)}{2}$
sum of all no. from 10 to 40 =
$\frac{40(40 + 1)}{2} - \frac{10(10 + 1)}{2}$
=820-55
=765