Đáp án + Giải thích các bước giải:
`1/41+1/42+...+1/79+1/80`
`=(1/41+1/42+...+1/60)+(1/61+1/62+...+1/80)`
Ta có : `1/41>1/60 ; 1/42>1/60 ; ... 1/59>1/60 `
`\to 1/41+1/42+...+1/60>1/60+1/60+...+1/60=20/60=1/3`
Lại có : `1/61>1/80 ; 1/62>1/80 ; .... ; 1/79>1/80 `
`\to 1/61+1/62+...+1/80>1/80+1/80+...+1/80=20/80=1/4`
`\to 1/41+1/42+...+1/60)+(1/61+1/62+...+1/80)>1/3+1/4=7/12`
Vậy `1/41+1/42+...+1/79+1/80>7/12`
