Đặt S = 1/10 + 1/15 + ... + 1/120
Ta có :
S = 1/10 + 1/15 + ... + 1/120
S = 2/20 + 2/30 + ... + 2/240
S = 2.(1/20 + 1/30 + ... + 1/240)
S = 2.(1/4.5 + 1/5.6 + ... + 1/15.16)
S = 2.(1/4 - 1/5 + 1/5 - 1/6 + ... + 1/15 - 1/16)
S = 2.(1/4 - 1/16)
S = 2. 3/16
S = 3/8