Câu trả lời:
Ta có:
1/20=1/20
1/19>1/20
1/18>1/20
.....
1/11>1/20
Suy ra 1/11+1/12+1/13+...+1/20>1/20*10=1/2. Vậy S>1/2.
Tương tự, ta có:
1/11=1/11
1/12<1/11
1/13<1/11
....
1/20<1/11
Suy ra 1/11+1/12+1/13+...+1/20<1/11*10=10/11<11/11=1. Vậy S < 1.
VẬY 1/2< S<1