algebra precalculus - Proof for formula for sum of sequence $1+2+3+ldots+n$?

Apparently $1+2+3+4+\ldots+n = \dfrac{n\times(n+1)}2$.

How? What's the proof? Or maybe it is self apparent just looking at the above?

PS: This problem is known as "The sum of the first $n$ positive integers".