Adding a sequence (1+2+3...+n)

1+2+3+4+...+10
An easy was to do this is (n*(n+1))/2 where n is the last number (so 10 in the above example)
so 1+2+3+4+...+10=55 or (10*11)/2
An easy trick (inside the trick) is that one of the multiplied numbers will be even...
Ex. 1+2+3+...+49 is the same as 49*25 or 49*(50/2)