Question

cho B= 1/4+1/5+1/6+....+1/19

chứng minh B>1

Answer 1

B = 1/4 + 1/5 + ...+1/19 > 1/4 + ( 1/20+1/20+..+1/20) = 1/4 + 3/4 = 1

=> B > 1

( chú ý: có 15 phân số 1/20)

Answer 2

Vì 1/4 >1/20 ; 1/5 > 1/20 ;...; 1/19 > 1/20

=>1/4 + 1/5 + 1/6 +...+ 1/19 > 1/20+1/20+1/20+...+1/20=10/20=1

=> đpcm

Answer 3

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