S=3/1.4 + 3/4.7 + 3/7.10 + ... + 3/40.43 + 3/43.46
= 1/3 ( 3/1.4 + 3/4.7 + 3/7.10 + ... + 3/40.43 + 3/43.46 )
= 1/3 ( 1/1.4 + 1/4.7 + 1/7.10 + ... + 1/40.43 + 1/43.46)
= 1/3 ( 1/1 - 1/4 + 1/4 -1/7 + ... + 1/40 -1/43 + 1/43 - 1/46)
= 1/3 . 45/46
= 15/46
Mà 15/46 < 1 nên ⇒ S<1
⇒đpcm