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sa se arate ca:4/5<1/3+1/4+1/5+1/6+1/8+1/9+1/10<3

Răspuns :

4/5 = 0.8
1/3+1/4+1/5+1/6+1/7+1/8+1/9+1/10 ≈ 1.42 <3
1/4 < 1/3 ; 1/5 < 1/3 ; 1/6 < 1/3..........1/10 < 1/3
S = 1/3 + 1/4 +1/5 +.........+1/10 < 8·1/3    S < 8/3 ⇒ S < 3    (9/3)
1/9 > 1/10 ; 1/8 > 1/10 ; .............1/3 > 1/10   
S = 1/3 +1/4 +.........+1/10  > 8·1/10      S > 8/10  ⇒  S > 4/5