Giải thích các bước giải:
Ta có: $7/12 = 4/12 + 3/12 = 1/3 + 1/4 = 20/60 + 20/80$
$1/41 + 1/42 + 1/43 +...+ 1/79 + 1/80$
$= (1/41 + 1/42 + 1/43 + ...+ 1/60) + (1/61 + 1/62 +...+ 1/79 + 1/80)$
Do $1/41> 1/42 > 1/43 > ...>1/59 > 1/60$
=>$ (1/41 + 1/42 + 1/43 + ...+ 1/60) > 1/60 + ...+ 1/60$
= $20/60 và 1/61> 1/62> ... >1/79> 1/80$
=>$ (1/61 + 1/62 +...+ 1/79 + 1/80) > 1/80 + ...+ 1/80$
= $20/80$
Vậy: $1/41 + 1/42 + 1/43 +...+ 1/79 + 1/80 > 20/60 + 20/80$
$= 7/12$
=> $1/41 + 1/42 + 1/43 +...+ 1/79 + 1/80 > 7/12 $
~ xin hay nhất ~
