1/2x1/3+1/3-1/4+1/4x1/5

= 1/2-1/3+1/3-1/4+1/4-1/5

=1/2-1/5

=3/10. (1/2x1/3= 1/2x3) 

1/2x1/3+1/3-1/4+1/4x1/5 = 1/2-1/3+1/3-1/4+1/4-1/5 =1/2-1/5 =3/10. (1/2x1/3= 1/2x3)

31/30 nhé, **** cho mình

`1/5xx1/2:3/10`

`=1/10xx10/3`

`=1/3`

`1/2xx1/3:1/4`

`=1/6xx4/1`

`=2/3`

a. \(\dfrac{1}{5}\times\dfrac{1}{2}:\dfrac{3}{10}=\dfrac{1}{10}:\dfrac{3}{10}=\dfrac{1}{10}\times\dfrac{10}{3}=\dfrac{10}{30}=\dfrac{1}{3}\)

b. \(\dfrac{1}{2}\times\dfrac{1}{3}:\dfrac{1}{4}=\dfrac{1}{6}:\dfrac{1}{4}=\dfrac{1}{6}\times\dfrac{4}{1}=\dfrac{4}{6}=\dfrac{2}{3}\)

a) \(\dfrac{1}{5}\times\dfrac{1}{2}:\dfrac{3}{10}=\dfrac{1}{5}\times\dfrac{1}{2}\times\dfrac{10}{3}=\dfrac{1\times1\times10}{5\times2\times3}=\dfrac{10}{30}=\dfrac{1}{3}\)

b) \(\dfrac{1}{2}\times\dfrac{1}{3}:\dfrac{1}{4}=\dfrac{1}{2}\times\dfrac{1}{3}\times4=\dfrac{1\times1\times4}{2\times3}=\dfrac{4}{6}=\dfrac{2}{3}\)

\(\frac{1}{1}\cdot\frac{1}{2}+\frac{1}{2}\cdot\frac{1}{3}+\frac{1}{3}\cdot\frac{1}{4}+...+\frac{1}{79}\cdot\frac{1}{80}\)

\(=\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+...+\frac{1}{79\cdot80}\)

\(=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}+...+\frac{1}{79}-\frac{1}{80}\)

\(=1-\frac{1}{80}\)

\(=\frac{79}{80}\)

a: =5-7/19

=95/19-7/19

=88/19

b: =1/2(3/4+1/4)=1/2

c: =26/7(11/3-8/3)

=26/7

`@Fù`

`=1/(1×2)+1/(2×3)+1/(3×4)+...+1/(72×73)`

`=1/1-1/2+1/2-1/3+...+1/72-1/73`

`=1-1/73`

`=72/73`

máy tính hoặc não