-1+2-3+4-5+6-7+8-....+100

Deret Bolak-Balik (Alternating Series).

-1 + 2 - 3 + 4 - 5 + 6 - 7 + 8 - ... + 100
= -(1 + 3 + 5 + 7 + ... + 99) + 2(1 + 2 + 3 + 4 + ... + 50)

Tinjau deret 1 + 3 + 5 + 7 + ... + 99.
Un = a + (n - 1)b
n = (Un - a) / b + 1
   = (99 - 1) / 2 + 1 = 50
Sn = n/2 [2a + (n - 1)b]
S₅₀ = 50/2 [2(1) + (50 - 1)(2)] = 2500

Tinjau deret 1 + 2 + 3 + 4 + ... + 50.
Un = a + (n - 1)b
n = (Un - a) / b + 1
   = (50 - 1) / 1 + 1 = 50
Sn = n/2 [2a + (n - 1)b]
S₅₀ = 50/2 [2(1) + (50 - 1)(1)] = 1275

Jadi:
-1 + 2 - 3 + 4 - 5 + 6 - 7 + 8 - ... + 100
= -(1 + 3 + 5 + 7 + ... + 99) + 2(1 + 2 + 3 + 4 + ... + 50)
= -2500 + 2(1275)
= 50