Question

B=1/4+1/5+1/6+...+1/19.       CMR B >1

 

Answer 1

Ta có:
\(\dfrac{1}{4}>\dfrac{1}{19}\)
\(\dfrac{1}{5}>\dfrac{1}{19}\)
\(\dfrac{1}{6}>\dfrac{1}{19}\)
\(...\)
\(\dfrac{1}{19}=\dfrac{1}{19}\)
\(\Rightarrow B=\dfrac{1}{4}+\dfrac{1}{5}+\dfrac{1}{6}+...+\dfrac{1}{19}>\dfrac{1}{19}+\dfrac{1}{19}+\dfrac{1}{19}+...+\dfrac{1}{19}=1\)  (19 SH \(\dfrac{1}{19}\))
\(\RightarrowĐPCM\)

\(\#PeaGea\)
 

Answer 2

Giúp tuiii

 

Answer 3

B = ( 1/4 + 1/5 + ... + 1/10) + ( 1/11 + 1/12 + ... + 1/19)

B > ( 1/10 + ... + 1/10) + ( 1/20 + ... + 1/20)

B > 7 x 1/10 + 9 x 1/20

= 7/ 10 + 9/20 

= 23/ 20

=> B > 20/20 > 1 => B >1