ta có: 1/32 <1/31<1/30 1/33<1/32<1/31 : : 1/61<1/60<1/59
___________________________________________________________________________________________
1/32+1/33+1/34+...+1/66<1/31+1/32+1/33+...+1/60<1/30+1/31+1/32+...+1/59
3/5 < 1/31+1/32+1/33+...+1/60< 4/5
=> 3/5< A<4/5