Để \(a\) là phân số

\(\Rightarrow n-7\ne0\\ \Rightarrow n\ne7\)

Vậy với \(n\ne7\) thì \(a\) là phân số 

no

mn. giúp em nha 

\(a.\dfrac{6}{5}=\dfrac{18}{x}\Rightarrow x=\dfrac{18\cdot5}{6}=15\\ \text{Vậy}\text{ }x=15.\)

\(b.\dfrac{3}{4}=\dfrac{-21}{x}\Rightarrow x=\dfrac{-21\cdot4}{3}=28\\ \text{ }\text{ }\text{ }\text{ }\text{Vậy }x=28.\)

\(c.\dfrac{x}{4}=\dfrac{21}{28}\Rightarrow x=\dfrac{21\cdot4}{28}=3\\ \text{Vậy }x=3.\)

\(d.\dfrac{-8}{2x}=\dfrac{3}{-9}\Rightarrow x=\dfrac{-8\cdot\left(-9\right)}{3}:2=12\\ \text{Vậy }x=12.\)

\(e.\dfrac{-4}{11}=\dfrac{x}{22}=\dfrac{40}{z}\\ \Rightarrow x=\dfrac{-4\cdot22}{11}=-8\\ \Rightarrow z=\dfrac{22\cdot40}{-8}=-110\\ \text{Vậy }x=-8;z=-110.\)

\(f.\dfrac{-3}{4}=\dfrac{x}{20}=\dfrac{21}{y}\\ \Rightarrow x=\dfrac{-3\cdot20}{4}=-15\\ \Rightarrow y=\dfrac{21\cdot20}{-15}=-28\\ \text{Vậy }x=-15;y=-28.\)

\(g.\dfrac{-4}{8}=\dfrac{x}{-10}=\dfrac{-7}{y}=\dfrac{z}{-24}\\ \Rightarrow x=\dfrac{-4\cdot\left(-10\right)}{8}=5\\ \Rightarrow y=\dfrac{-7\cdot\left(-10\right)}{5}=14\\ \Rightarrow z=\dfrac{-7\cdot\left(-24\right)}{14}=12\\ \text{Vậy }x=5;y=14;z=12.\)

\(h.\dfrac{x}{4}=\dfrac{9}{x}\\ \Rightarrow x\cdot x=9\cdot4\\ \Rightarrow x\cdot x=36\\ \Rightarrow x\cdot x=6\cdot6\\ \text{Vậy }\text{cả hai }x=6.\)

`3x+2(3-2x)=-5`

`3x+6-4x=-5`

`3x-4x=-5-6`

`-x=-11`

`x=11`

cảm ơn nha

\(4^2.x+2^4.x+3=144\\ \Rightarrow16x+16x+3=144\\ \Rightarrow32x=144-3\\ \Rightarrow32x=141\\ \Rightarrow x=\dfrac{141}{32}\)

2x + 2x + 3 = 144

         =>  2x . 1 + 2x . 23 = 144 

          => 2x . (1 + 23) = 144

         =>  2x . 9 = 144

         => 2x = 144 : 9 = 16

        => x = 4

Ta có 6a + 11b chia hết cho 31
Vậy: 6a + 42b - 31b = 6x(a+7b) - 31xb chia hết cho 31
nên: 6x(a + 7b) chia hết cho 31
Do vậy: a + 7b chia hết cho 31 (đpcm)

\(\dfrac{7}{9}+\dfrac{1}{3} < x < \dfrac{43}{8}+\dfrac{1}{10}\)

\(\dfrac{10}{9} < x < \dfrac{219}{40}\)

  Mà \(x \in N\)

  \(=>x=\){`2;3;4;5`}

\(\dfrac{7}{9}+\dfrac{1}{3}< x< \dfrac{43}{8}+\dfrac{1}{10}\)

\(\dfrac{10}{9}< x< \dfrac{219}{40}\)

Mà \(x\inℕ\)

\(\Rightarrow\dfrac{10}{9}< 2\le x\le5< \dfrac{219}{40}\)

\(\Rightarrow2\le x\le5\)

\(\Rightarrow x\in\left\{2;3;4;5\right\}\)

Vậy: \(x\in\left\{2;3;4;5\right\}\)