Question

1+2=3+4+5+.............100=?? give the formulae.

Answer 1

sum=[n*(n+1)]/2
where, 'n' is the maximum value of the series.
here it is 100
hope this helps..  :)

Answer 2

$\frac{n(n+1)}{2}$ is the formula..these are common numbers...n=number of terms