a) \(\dfrac{2}{5}\cdot x=\dfrac{1}{2}+\dfrac{6}{8}\)

    \(\dfrac{2}{5}\cdot x=\dfrac{5}{4}\)

          \(x=\dfrac{5}{4}\div\dfrac{2}{5}\)

          \(x=\dfrac{25}{8}\)

b) \(\dfrac{20}{7}-x=\dfrac{19}{7}\div\dfrac{3}{2}\)

    \(\dfrac{20}{7}-x=\dfrac{19}{7}\cdot\dfrac{2}{3}\)

    \(\dfrac{20}{7}-x=\dfrac{38}{21}\)

             \(x=\dfrac{20}{7}-\dfrac{28}{21}\)

             \(x=\dfrac{22}{21}\)

a) x=25/8

b)=22/21

a) x=25/8

b)=22/21

Bài 1

2/5 - 3/5 : (0,75 + 30%)

= 2/5 - 3/5 : 21/20

= 2/5 - 4/7

= -6/35

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Bài 2

a) x/6 = -11/3

x = -11/3 . 6

x = -22

b) 16/5 của x là 48

x = 48 : 16/5

x = 15

bài1

2/5-3/5:(0,75+30%)=2/5-3/5:(75/100+30/100)

                               =2/5-3/5:105/100

                               =2/5-3/5:21/20

                               = 2/5-3/5. 20/21

                               =2/5-1/1.4/7

                               =2/5-4/7

                               =2/5+-4/7

                               =14/35+-20/35

                               =34/35

\(a,\Leftrightarrow x^2+6x+9-x^2+3x+10=1\\ \Leftrightarrow9x=-18\Leftrightarrow x=-2\\ b,\Leftrightarrow4x^2-4x+1-4x^2+17x+15=3\\ \Leftrightarrow13x=-13\Leftrightarrow x=-1\\ c,\Leftrightarrow3x\left(x-2\right)+4\left(x-2\right)=0\\ \Leftrightarrow\left(3x+4\right)\left(x-2\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=-\dfrac{4}{3}\\x=2\end{matrix}\right.\\ d,\Leftrightarrow2x\left(3x+5\right)-6\left(3x+5\right)=0\\ \Leftrightarrow\left(x-3\right)\left(3x+5\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=3\\x=-\dfrac{5}{3}\end{matrix}\right.\)

b: \(\Leftrightarrow48x^2-12x-20x+5-48x^2+36x=30\)

\(\Leftrightarrow4x=25\)

hay \(x=\dfrac{25}{4}\)

5/8

B 5/8 nha 

B 5/8

\(x:\dfrac{13}{16}=\dfrac{5}{-8}\\ x=\dfrac{5}{-8}.\dfrac{13}{16}=\dfrac{65}{-128}\)

\(x.\dfrac{-1}{2}=\dfrac{-4}{5}\\ x=\dfrac{-4}{5}:\dfrac{-1}{2}=\dfrac{8}{5}\)

b: \(5x^2+3x-2-4x^2+x+5=0\)

\(\Leftrightarrow\left(x+1\right)\left(x+3\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x=-1\\x=-3\end{matrix}\right.\)

a) \(\Rightarrow x^8-2x^4-8=0\Rightarrow\left(x^4-4\right)\left(x^4+2\right)=0\)

\(\Rightarrow\left(x^2-2\right)\left(x^2+2\right)\left(x^4+2\right)=0\)

\(\Rightarrow x^2=2\Rightarrow x=\pm\sqrt{2}\)(do \(x^2+2\ge2>0,x^4+2\ge2>0\))

b) \(\Rightarrow x^2+4x+3=0\Rightarrow\left(x+1\right)\left(x+3\right)=0\)

\(\Rightarrow\left[{}\begin{matrix}x=-1\\x=-3\end{matrix}\right.\)

\(a,\Leftrightarrow x^8-2x^4-8=0\\ \Leftrightarrow x^8+2x^4-4x^4-8=0\\ \Leftrightarrow\left(x^4+2\right)\left(x^4-4\right)=0\\ \Leftrightarrow\left(x^4+2\right)\left(x^2-2\right)\left(x^2+2\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=\sqrt{2}\\x=-\sqrt{2}\end{matrix}\right.\left(x^2+2>0;x^4+2>0\right)\\ b,\Leftrightarrow x^2+4x+3=0\\ \Leftrightarrow\left(x+1\right)\left(x+3\right)=0\Leftrightarrow\left[{}\begin{matrix}x=-1\\x=-3\end{matrix}\right.\)

a: \(x+6\dfrac{1}{8}=8\)

=>\(x+\dfrac{49}{8}=\dfrac{64}{8}\)

=>\(x=\dfrac{64}{8}-\dfrac{49}{8}=\dfrac{15}{8}\)

b: \(\dfrac{11}{2}\cdot x=\dfrac{1}{5}:\dfrac{1}{3}\)

=>\(x\cdot\dfrac{11}{2}=\dfrac{1}{5}\cdot3=\dfrac{3}{5}\)

=>\(x=\dfrac{3}{5}:\dfrac{11}{2}=\dfrac{3}{5}\cdot\dfrac{2}{11}=\dfrac{6}{55}\)

c: \(x\cdot\dfrac{3}{5}+\dfrac{2}{5}\cdot x=\dfrac{4}{9}+\dfrac{1}{3}\)

=>\(x\left(\dfrac{3}{5}+\dfrac{2}{5}\right)=\dfrac{4}{9}+\dfrac{3}{9}\)

=>\(x\cdot1=\dfrac{7}{9}\)

=>\(x=\dfrac{7}{9}\)

\(\left(x+5\right)^2-\left(x-3\right)\left(x+7\right)=19\\ x^2+10x+25-\left(x^2+7x-3x-21\right)=19\\ x^2+10x+25-x^2-7x+3x+21=19\\ 6x+46=19\\ 6x=19-46\\ 6x=-27\\ x=\dfrac{-27}{6}\\ x=-4,5\)

Chọn D

D