Đáp án+Giải thích các bước giải:
`S = (1/31+1/32+1/33+...+1/40) + (1/41 + 1/42 + ...+ 1/50) + (1/51 + 1/52+...+1/59+1/60)`
Mà:
`@ (1/31+1/32+1/33+...+1/40) > 1/40 × 10 = 1/4`
`@ (1/41 + 1/42 + ...+ 1/50) > 1/5 ; (1/51 + 1/52+...+1/59+1/60) > 1/6`
→ `S > 1/4 + 1/5 + 1/6`
`@ (1/4 + 1/5 + 1/6) > 3/5`
`→ S > 3/5` (1)
Ta có:
`(1/31+1/32+1/33+...+1/40) < 1/31 × 10 = 10/30 = 1/3`
`→ S < 4/5` (2)
Từ `(1)` và `(2)`, ta có:
`3/5<S<4/5`
