\(\begin{array}{l}a)\frac{x}{{ - 3}} = \frac{7}{{0,75}}\\ \Rightarrow x.0,75 = ( - 3).7\\ \Rightarrow x = \frac{{( - 3).7}}{{0,75}} =  - 28\end{array}\)

Vậy x = 28

\(\begin{array}{l}b) - 0,52:x = \sqrt {1,96} :( - 1,5)\\ - 0,52:x = 1,4:( - 1,5)\\ x = \dfrac{(-0,52).(-1,5)}{1,4}\\x = \frac{39}{{70}}\end{array}\)

Vậy x = \(\frac{39}{{70}}\)

\(\begin{array}{l}c)x:\sqrt 5  = \sqrt 5 :x\\ \Leftrightarrow \frac{x}{{\sqrt 5 }} = \frac{{\sqrt 5 }}{x}\\ \Rightarrow x.x = \sqrt 5 .\sqrt 5 \\ \Leftrightarrow {x^2} = 5\\ \Leftrightarrow \left[ {_{x =  - \sqrt 5 }^{x = \sqrt 5 }} \right.\end{array}\)

Vậy x \( \in \{ \sqrt 5 ; - \sqrt 5 \} \)

Chú ý:

Nếu \({x^2} = a(a > 0)\) thì x = \(\sqrt a \) hoặc x = -\(\sqrt a \)

a: \(\dfrac{x}{-3}=\dfrac{7}{0.75}=\dfrac{28}{3}\)

=>\(x=\dfrac{28\left(-3\right)}{3}=-28\)

b: \(-\dfrac{0.52}{x}=\dfrac{\sqrt{1.96}}{-1.5}=\dfrac{1.4}{-1.5}\)

=>\(x=0.52\cdot\dfrac{1.5}{1.4}=\dfrac{39}{70}\)

c: \(\dfrac{x}{\sqrt{5}}=\dfrac{\sqrt{5}}{x}\)

=>\(x^2=5\)

=>\(x=\pm\sqrt{5}\)

a,\(\frac{25x}{3}=\frac{\frac{4}{7}}{\frac{2}{7}}=7\)=> x=\(\frac{21}{25}\)

b,\(\frac{0,15}{3\sqrt{x}}=\frac{0,3\cdot3}{2}\)<=>\(0,3=0,3\cdot9\sqrt{x}\)Hay \(\sqrt{x}=\frac{1}{9}=>x=\frac{1}{3}\)

\(\frac{1}{3}x:\frac{2}{3}=1\frac{3}{4}:\frac{2}{5}\)

\(\Rightarrow\frac{1}{3}x:\frac{2}{3}=\frac{35}{8}\)

\(\Rightarrow\frac{1}{3}x=\frac{35}{12}\)

\(\Rightarrow x=\frac{35}{4}\)

a) \(\left(\frac{1}{3}\cdot x\right):\frac{2}{3}=1\frac{3}{4}:\frac{2}{5}\)

\(=\left(\frac{1}{3}\cdot x\right):\frac{2}{3}=\frac{35}{8}\)

\(\Rightarrow\frac{1}{3}\cdot x=\frac{35}{8}\cdot\frac{2}{3}\)

\(\Rightarrow x=\frac{35}{12}:\frac{1}{3}=\frac{35}{4}\)

b) \(4,5:0,3=2,5:\left(0,1\cdot x\right)\)

\(=15=2,5:\left(0,1\cdot x\right)\)

\(\Rightarrow0,1\cdot x=2,25:15\)

\(\Rightarrow x=\frac{3}{20}:0,1=\frac{3}{2}=1,5\)

c) \(8:\left(\frac{1}{4}\cdot x\right)=2:0,02\)

\(8:\left(\frac{1}{4}\cdot x\right)=100\)

\(\Rightarrow\frac{1}{4}\cdot x=8:100\)

\(\Rightarrow x=\frac{2}{25}:\frac{1}{4}=\frac{8}{25}=0,32\)

d) \(3:2\frac{1}{4}=\frac{3}{4}:\left(6\cdot x\right)\)

\(=\frac{4}{3}=\frac{3}{4}:\left(6:x\right)\)

\(\Rightarrow6\cdot x=\frac{3}{4}:\frac{4}{3}\)

\(\Rightarrow x=\frac{9}{16}:6=\frac{3}{32}=0,09375\)

x/0,3 = 7,5/x

=>x^2 = 2,25

=>x^2 = (1,5)^2 hoặc x = (-1,5)^2

=>x = 1,5         hoặc x = -1,5

x/0.3=7.5/x

=>x^2=7.5*0.3

=>x^2=2.25

=>x=+-1.5

413 :x4 =60,3 

⇒133 :x4 =20

⇒x4 =133 :20

⇒x4 =1360 

⇒x=1360 .4

⇒x=1315 

<=> 13/3 : x/4 = 20

<=> x/4 = 13/3 : 20 = 13/60

<=> x = 13/60 . 4 = 13/15

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Vì (-0,4) : x = 1,2 : 0,3 nên \(\frac{{ - 0,4}}{x} = \frac{{1,2}}{{0,3}} \Rightarrow ( - 0,4).0,3 = 1,2.x \Rightarrow x = \frac{{( - 0,4).0,3}}{{1,2}} =  - 0,1\)

Vậy x = - 0,1

a) \(\left(\frac{1}{3}.x\right):\frac{2}{3}=\frac{7}{4}:\frac{2}{5}\)

\(\left(\frac{1}{3}.x\right):\frac{2}{3}=\frac{35}{8}\)

\(\Rightarrow\frac{1}{3}.x=\frac{35}{8}.\frac{2}{3}\)

\(\Rightarrow\frac{1}{3}.x=\frac{35}{12}\)

\(\Rightarrow x=\frac{35}{12}:\frac{1}{3}\)

\(\Rightarrow x=\frac{35}{4}\)

Vậy \(x=\frac{35}{4}\)

a) x= \(\frac{35}{4}\)

\(a.\left(\frac{1}{3}.x\right):\frac{2}{3}=\frac{7}{4}:\frac{2}{5}\)

    \(\left(\frac{1}{3}.x\right):\frac{2}{3}=\frac{35}{8}\)

          \(\left(\frac{1}{3}.x\right)=\frac{35}{8}.\frac{2}{3}\)

          \(\left(\frac{1}{3}.x\right)=\frac{35}{12}\)

                    \(x=\frac{35}{12}:\frac{1}{3}\)

                    \(x=\frac{35}{4}\)

\(b.4,5:0,3=2,25:\left(0,1.x\right)\)

\(\Rightarrow2,25:\left(0,1.x\right)=4,5:0,3\)

     \(2,25:\left(0,1.x\right)=15\)

     \(2,25:\left(0,1.x\right)=2,25:15\)

                \(\left(0,1.x\right)=0,15\)

                          \(x=0,15:0,1\)

                          \(x=1,5\)

\(c.8:\left(\frac{1}{4}.x\right)=2:0,02\)

   \(8:\left(\frac{1}{4}.x\right)=100\)

        \(\left(\frac{1}{4}.x\right)=8:100\)

        \(\left(\frac{1}{4}.x\right)=\frac{2}{25}\)

                  \(x=\frac{2}{25}:\frac{1}{4}\)

                  \(x=\frac{8}{25}\)

\(d.3:\frac{9}{4}=\frac{3}{4}:\left(6.x\right)\)

\(\Rightarrow\frac{3}{4}:\left(6.x\right)=3:\frac{9}{4}\)

     \(\frac{3}{4}:\left(6.x\right)=\frac{4}{3}\)

           \(\left(6.x\right)=\frac{3}{4}:\frac{4}{3}\)

          \(\left(6.x\right)=\frac{9}{16}\)

                \(x=\frac{9}{16}:6\)

                \(x=\frac{3}{32}\)

 ^...^ ^_^