Ta có :7/12=4/12+3/12=1/3+1/4 1/41>1/60;1/42>1/60;=--..;1/59>1/60 =>1/41+1/42+=...+1/60>1/60+1/60+--..+1/60=1/60*20=1/3 (1) 1/61>1/80;1/62>1/80;===-.;1/79>1/80 =>1/61+/162+=-...+1/80>1/80+1/80+1/80+=+1/80=1/80*20=1/4 (2) Từ (1) và (2) =>1/41+1/42=-+1/80>1/3+1/4=-7/12 Vậy 1/41+1/42+=+1/80>7/12