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

a = -100
b = -99 - (-100) = 1
Un = 50

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

Sn = n/2 (a + Un)
S151 = 151/2 ((-100) + 50)
S151 = 151/2 . (-100)
S151 = 151 . (-50)
S151 = -7550

Semoga membantu dan semoga bermanfaat...