rumus umum 1+2+3+...+n =... pakai cara ya kak,thx

n = 1,2,3,4.........dst
(n-1 )= 0,1,2,3,4.....

Sn = (a + 0.b )(a+1.b) + (a +2.b) .........U(n-2) + U(n-1) + Un
Sn = (Un + U(n-1) + U(n-2).......+ a
==========================+
Sn = ( a + Un) x n/2