-100-99-98-...-2-1-0+1+2+...+48+49+50

-100 - 99 - 98 - ... + 50

U₁ = a = -100
  b = -99 - (-100)
     = -99 + 100
     = 1

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

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

semoga jelas dan membantu ya