Ta có:
`1/1000 + 13/1000 + 25/1000 + 37/1000 + 49/1000 + ... + 87/1000 + 99/1000`
`= (1 + 13 + 25 + 37 + 49 + ... + 87 + 99)/1000`
Đặt:
`A = 1 + 13 + 25 + 37 + 49 + ... + 87 + 99`
`A = (1+ 99) + (13 + 87) + (25 + 75) + (37 + 63) + (49 + 51)`
`A = 100 + 100 + 100 + 100 + 100`
`A = 100 × 5`
`A = 500`
Nên:
`1/1000 + 13/1000 + 25/1000 + 37/1000 + 49/1000 + ... + 87/1000 + 99/1000`
`= (1 + 13 + 25 + 37 + 49 + ... + 87 + 99)/1000`
`= 500/1000`
`= 1/2`