tính nhanh : 1/1000+13/1000+25/1000+37/1000+49/1000+.......+87/1000+99/1000

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`