Question

cho \(B=\frac{1}{4}+\frac{1}{5}+\frac{1}{6}+...+\frac{1}{19}\)

chung to rang B >1

Answer 1

Ta có: 1/4+1/5+...+1/10>1/10.7=7/10

1/11+1/12+...+1/19>1/20.9=9/20

Kết hợp lại ta có B= 1/4+1/5+1/6+...+1/19>7/10+9/20=23/20>1.Vậy B>1

Answer 2

ta co 1/4+1/5+......+1/10>1/10.7=7/10

1/11+1/12+.....1/19>1/20.9=9/20

kết hợp lại ta có mB=1/4+1/5+1/6+......1/19>7/10+9/20=23/20>1 vậy B>1

Answer 3

ta có:

1/4>1/10

1/5>1/10

1/6>1/10

1/7>1/10

1/8>1/10

1/9>1/10

=>1/4 +1/5+1/6+1/7+1/8+1/9>1/10*6

=>1/4+1/5+1/6+1/7+1/8+1/9+1/10>1/10*7

=>1/4+1/5+1/6+...+1/19>1/10*7+1/11+1/12+...+1/19

=>1/4+1/5+1/6+...+1/19>7/10+1/11+1/12+...+1/19

mà ta thấy:

1/11>1/20

1/12>1/20

1/13>1/20

1/14>1/20

1/15>1/20

...

1/19>1/20

=>1/11+1/12+1/13+...+1/19>1/20*9

=>1/4+1/5+1/6+...+1/19>1/10*7+1/20*9

=>B>7/10+9/20

=>B>23/20=1.15>1

=>B>1(dpcm)

Answer 4

Ko ghi lại đề 

\(=\frac{1}{10}>\frac{1}{10}.7=\frac{7}{10}\)

\(=\frac{1}{11}+\frac{1}{12}+......\frac{1}{19}>\frac{1}{20}.9=\frac{9}{20}\)

B= 1/4 + 1/5 + 1/6 +.............1/19>7/10 +9/20 = 23/20 ?1 

Vậy B >1

Answer 5

helo các bạn