MSC : `9`

`2/9` giữ nguyên

`2/3=(2xx3)/(3xx3)= 6/9`

`x:(-5/3) -(-2/5) =3/10`

`=> x: (-5/3) + 2/5 =3/10`

`=> x:(-5/3) = 3/10-2/5`

`=>x:(-5/3)= 3/10 - 4/10`

`=> x:(-5/3) = -1/10`

`=> x= -1/10 * (-5/3)`

`=> x= 5/30`

`=>x= 1/6`

Vậy `x=1/6`

ờ ừ=))

Bài `1`

\(\dfrac{14}{28}=\dfrac{14:14}{28:14}=\dfrac{1}{2}\\ \dfrac{25}{50}=\dfrac{25:25}{50:25}=\dfrac{1}{2}\\ \dfrac{48}{30}=\dfrac{48:6}{30:6}=\dfrac{8}{5}\\ \dfrac{81}{54}=\dfrac{81:27}{54:27}=\dfrac{3}{2}\)

Bài `2`

`20/30=(20:10)/(30:10)= 2/3`

`8/12=(8:4)/(12:4)=2/3`

Bài `3`

`25/100=(25:5)/(100:5)=5/20`

Bài `4`

`b,(8xx7xx5)/(11xx8xx7)=5/11`

`c,(19xx2xx5)/(19xx3xx5)=2/3`

\(\dfrac{7}{8}+\dfrac{7}{120}+\dfrac{7}{360}+\dfrac{7}{\left(7n-6\right)\left(7n+1\right)}+\dfrac{1}{7n+1}\\ =\left(\dfrac{7}{8}+\dfrac{7}{120}+\dfrac{7}{360}\right)+\left(\dfrac{7}{\left(7n-6\right)\left(7n+1\right)}+\dfrac{1}{7n+1}\right)\\ =\left(\dfrac{315}{360}+\dfrac{21}{360}+\dfrac{7}{360}\right)+\left(\dfrac{7}{\left(7n-6\right)\left(7n+1\right)}+\dfrac{7n-6}{\left(7n+1\right)\left(7n-6\right)}\right)\)

\(=\dfrac{343}{360}+\dfrac{7n+1}{\left(7n-6\right)\left(7n+1\right)}\\ =\dfrac{343}{360}+\dfrac{1}{7n-6}\\ =\dfrac{343\left(7n-6\right)+360}{360\left(7n-6\right)}\\ =\dfrac{2401n-2058+360}{360\left(7n-6\right)}\\ =\dfrac{2401n-1698}{360\left(7n-6\right)}\)

`x^2(x-5)-4(5-x)=0`

\(\Leftrightarrow x^2\left(x-5\right)+4\left(x-5\right)=0\\ \Leftrightarrow\left(x-5\right)\left(x^2+4\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x-5=0\\x^2+4=0\end{matrix}\right.\\ \Leftrightarrow\left[{}\begin{matrix}x=5\\x^2=-4\end{matrix}\right.\\ \Leftrightarrow\left[{}\begin{matrix}x=5\\x\in\varnothing\end{matrix}\right.\)