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

A + B = 1 - 2 + 3 - 4 + 5 - ... + 99 - 100

Ada 2 deret :
A = 1 + 3 + 5 + ... + 99
n = 50
Sn = (n/2)(a + Un)
A = S50 = (50/2)(1 + 99)
S50 = 2500

B = -2 - 4 - 6 - ... - 100
n = 50
B = S50 = (50/2)(-2 - 100)
S50 = -2550

A + B
= 2500 + (-2550)
= -50

Cara lain :
(1 - 2) + (3 - 4) + ... + (99 - 100)
= (-1) × 100/2
= -50

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