-100-99-98-...-2-1-0+1+2...+48+49+50 tanpa menghitung satu persatu

a = -100
b = 1

Un = a + (n-1)b
50 = -100 + (n-1)1
50 = -100 + n - 1
50 = n - 101
n = 50 + 101
n= 151

Sn = n/2(a+Un)
S151 = 151/2 (-100 + 50)
          = 151/2 x (-50)
          = - 3.775