Question

Answer 1

nhanh nhé

Answer 2

làm bài 4 thôi

 

Answer 3

Bài 1: 

a) Ta có: \(\left(\dfrac{-1}{6}+\dfrac{5}{-12}\right)+\dfrac{7}{12}\)

\(=\dfrac{-2}{12}+\dfrac{-5}{12}+\dfrac{7}{12}\)

=0

b) Ta có: \(\dfrac{7}{36}-\dfrac{8}{-9}+\dfrac{-2}{3}\)

\(=\dfrac{7}{36}+\dfrac{8}{9}+\dfrac{-2}{3}\)

\(=\dfrac{7}{36}+\dfrac{32}{36}+\dfrac{-24}{36}\)

\(=\dfrac{15}{36}=\dfrac{5}{12}\)

Answer 4

Bài 4: 

Ta có: \(\dfrac{1}{50}>\dfrac{1}{100}\)

\(\dfrac{1}{51}>\dfrac{1}{100}\)

...

\(\dfrac{1}{99}>\dfrac{1}{100}\)

Do đó: \(\dfrac{1}{50}+\dfrac{1}{51}+...+\dfrac{1}{99}>\dfrac{1}{100}+\dfrac{1}{100}+...+\dfrac{1}{100}\)

\(\Leftrightarrow S>\dfrac{51}{100}\)

mà \(\dfrac{51}{100}>\dfrac{50}{100}\)

nên \(S>\dfrac{50}{100}\)

hay \(S>\dfrac{1}{2}\)(đpcm)