Đáp án:
Giải thích các bước giải:
`1/41>1/60;1/42>1/60;...;1/59>1/60`
`=>1/41+1/42+...+1/60>1/60+1/60+...+1/60` (Có `20` số hạng).
`=>1/41+1/42+...+1/60>1/3`
`=>1/61>1/80;1/62>1/80;...;1/79>1/80`
`=>1/61+1/62+...+1/80>1/80+1/80+...+1/80` (Có `20` số hạng)
`=>1/61+1/62+....+1/80>1/4`
`=>1/41+1/42+....+1/80>1/3+1/4=7/12`
`=>1/40>1/41;1/40>1/42;...;1/40>1/60`
`=>1/41+1/42+...+1/60<1/40+1/40+...+1/60`
`=>1/41+1/42+...+1/60<1/2`
`=>1/61<1/60;1/62<1/60;...;1/80<1/60`
`=>1/61+1/62+...+1/80<1/60+1/60+...+1/60`
`=>1/61+1/62+...+1/80<1/3`
`=>1/41+1/42+1/43+....+1/80<1/2+1/3=5/6`
Vậy `7/12<A<5/6`.
