Đề là gì bạn

C = 1 / 10 + ( 1 / 11 + 1 / 12 + ... + 1 / 99 + 1 / 100 )          

      <=>  1 / 10 + ( 1 / 11 + 1 / 12 + ... + 1 / 99 + 1 / 100 ) >  1 / 10 + ( 1 / 100 + 1 / 100 + ... + 1 / 100 )  

         <=> 1/ 10 + 90 / 100 = 1       

                       Vậy C > 1 (đpcm)

C = 1 / 10 + ( 1 / 11 + 1 / 12 + ... + 1 / 99 + 1 / 100 )          

      <=>  1 / 10 + ( 1 / 11 + 1 / 12 + ... + 1 / 99 + 1 / 100 ) >  1 / 10 + ( 1 / 100 + 1 / 100 + ... + 1 / 100 )  

         <=> 1/ 10 + 90 / 100 = 1       

                       Vậy C > 1 (đpcm)

Ta có : 

Cần 30 số hạng đầu đã lớn hơn 1.  

1/10+1/11+…+1/19 > 1/20+1/20+…+1/20 = 10/20 = 1/2  

1/20+1/21+…+1/29 > 1/30+1/30+…+1/30 = 10/30 = 1/3  

1/30+1/31+…+1/39 > 1/40+1/40+…+1/40 = 10/40 = 1/4  

=>  1/10+1/11+…+1/39 > 1/2+1/3+1/4 = 13/12 > 1

​Vậy :C>1

sao lại chọn người không tự làm mà đi copy ?      

C>1   vì c>1

a, Ta có: \(A=\frac{1}{11}+\frac{1}{12}+...+\frac{1}{50}=\left(\frac{1}{11}+\frac{1}{12}+...+\frac{1}{30}\right)+\left(\frac{1}{31}+\frac{1}{32}+...+\frac{1}{60}\right)\)

Nhận xét: \(\frac{1}{11}+\frac{1}{12}+....+\frac{1}{30}>\frac{1}{30}+\frac{1}{30}+...+\frac{1}{30}=\frac{20}{30}=\frac{2}{3}\)

\(\frac{1}{31}+\frac{1}{32}+...+\frac{1}{60}>\frac{1}{60}+\frac{1}{60}+...+\frac{1}{60}=\frac{20}{60}=\frac{1}{3}\)

\(\Rightarrow A>\frac{2}{3}+\frac{1}{3}=1>\frac{1}{2}\)

Vậy A > 1/2

b, Ta có: \(\frac{1}{50}>\frac{1}{100};\frac{1}{51}>\frac{1}{100};........;\frac{1}{99}>\frac{1}{100}\)

\(\Rightarrow B>\frac{1}{100}+\frac{1}{100}+...+\frac{1}{100}=\frac{50}{100}=\frac{1}{2}\)

Vậy B > 1/2

c, Ta có: \(C=\frac{1}{10}+\frac{1}{11}+\frac{1}{12}+...+\frac{1}{100}=\frac{1}{10}+\left(\frac{1}{11}+\frac{1}{12}+...+\frac{1}{100}\right)\)

Nhận xét: \(\frac{1}{11}+\frac{1}{12}+...+\frac{1}{100}>\frac{1}{100}+\frac{1}{100}+...+\frac{1}{100}=\frac{90}{100}=\frac{9}{10}\)

\(\Rightarrow C>\frac{1}{10}+\frac{9}{10}=\frac{10}{10}=1\)

Vậy C > 1

@Miyuki Misaki, @Nguyễn Trúc Giang, @Nguyễn Lê Phước Thịnh, @White Hold

ta thay tu 1/10 +1/11+............+1/20 >1/20+1/20+........+1/20 =10/20=1/2 

1/21+............+1/30 >1/30+1/30+........+1/30 =10/30=1/3

1/31 +1/32+............+1/40 >1/40+1/40+........+1/40 =10/40=1/4

=>1/10+1/11+-----+1/40>1/2+1/3+1/4=13/12>1

=>a>1 

Chỉ cần 30 số hạng đầu đã lớn hơn 1. 
1/10+1/11+…+1/19 > 1/20+1/20+…+1/20 = 10/20 = 1/2 
1/20+1/21+…+1/29 > 1/30+1/30+…+1/30 = 10/30 = 1/3 
1/30+1/31+…+1/39 > 1/40+1/40+…+1/40 = 10/40 = 1/4 
=> 1/10+1/11+…+1/39 > 1/2+1/3+1/4 = 13/12 > 1

Ta có:\(\frac{1}{10}+\frac{1}{11}+...+\frac{1}{100}\)> \(\frac{1}{10}+\left(\frac{1}{100}+\frac{1}{100}+...+\frac{1}{100}\right)\)[ 90 p/s \(\frac{1}{100}\)]

= \(\frac{1}{10}+\frac{90}{100}=\frac{10}{100}+\frac{90}{100}\)=\(\frac{100}{100}=1\)

Vậy \(\frac{1}{10}+\frac{1}{11}+...+\frac{1}{100}\)>1

\(\frac{1}{10}+\frac{1}{11}+...+\frac{1}{100}\)\(\frac{1}{10}+\frac{1}{11}+...+\frac{1}{100}\)