Question

1/41+1/42+1/43+...+1/79 va 7/12( So sanh )

lam nhanh minh tick cho5 cai nha

Answer 1

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 + 1/63 + ...+ 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>..1/80

( 1/61 + 1/62 + 1/63 + ...+ 1/80 ) > 1/80 +....+1/80 = 20/80

vậy 1/41 + 1/42 + 1/43 + .... + 1/79 + 1/80 > 20/60 + 20/80

1/41 + 1/42 + 1/43 + ..... + 1/79 + 1/80 > 7/12