90 x 2 = 180

80 x 2 = 160

70 x 2 = 140

60 x 2 = 120

90 x 2 = 180

80 x 2 = 160

70 x 2 = 140

60 x 2 = 120

=189

=160

=140

=120

k mik nha

a; 

c; \(\dfrac{1}{10}\) + \(\dfrac{4}{20}\) + \(\dfrac{9}{30}\)+\(\dfrac{16}{40}+\dfrac{25}{50}+\dfrac{36}{60}+\dfrac{49}{70}+\dfrac{64}{80}+\dfrac{81}{90}\)

= \(\dfrac{1}{10}+\dfrac{2}{10}+\dfrac{3}{10}+\dfrac{4}{10}+\dfrac{5}{10}+\dfrac{6}{10}+\dfrac{7}{10}+\dfrac{8}{10}+\dfrac{9}{10}\)

= \(\dfrac{1+2+3+4+5+6+7+8+9}{10}\)

=  \(\dfrac{\left(1+9\right)+\left(2+8\right)+\left(3+7\right)+\left(4+6\right)+5}{10}\)

= \(\dfrac{10+10+10+10+5}{10}\)

= \(\dfrac{\left(10+10+10+10\right)+5}{10}\)

= \(\dfrac{10\times4+5}{10}\)

= \(\dfrac{45}{10}\)

= \(\dfrac{9}{2}\)

Bài 1:

a; (\(\dfrac{1}{4}\)\(x\) - \(\dfrac{1}{8}\)) x \(\dfrac{3}{4}\) = \(\dfrac{1}{4}\)

     \(\dfrac{1}{4}x\) - \(\dfrac{1}{8}\) = \(\dfrac{1}{4}\) : \(\dfrac{3}{4}\)

      \(\dfrac{1}{4}\)\(x\) - \(\dfrac{1}{8}\) = \(\dfrac{1}{4}\) x \(\dfrac{4}{3}\)

    \(\dfrac{1}{4}x\) - \(\dfrac{1}{8}\) =  \(\dfrac{1}{3}\)

      \(\dfrac{1}{4}x\) = \(\dfrac{1}{3}\) + \(\dfrac{1}{8}\)

       \(\dfrac{1}{4}\) \(x\)=  \(\dfrac{8}{24}\) + \(\dfrac{11}{24}\)

          \(\dfrac{1}{4}x=\dfrac{11}{24}\)

           \(x=\dfrac{11}{24}:\dfrac{1}{4}\)

           \(x=\dfrac{11}{24}\times4\)

           \(x=\dfrac{11}{6}\) 

b; \(\dfrac{12}{5}:x\) = \(\dfrac{14}{3}\) x \(\dfrac{4}{7}\)

     \(\dfrac{12}{5}\) : \(x\) = \(\dfrac{8}{3}\)

            \(x\) = \(\dfrac{12}{5}\) : \(\dfrac{8}{3}\)

            \(x\) = \(\dfrac{12}{5}\) x \(\dfrac{3}{8}\)

             \(x\) = \(\dfrac{9}{10}\)

c; \(x+\dfrac{2}{3}\) = 8:4 - 1

   \(x+\dfrac{2}{3}\) = 2 - 1

   \(x+\dfrac{2}{3}\) = 1

  \(x=1-\dfrac{2}{3}\)

  \(x\) = \(\dfrac{1}{3}\)

a) 86 – 16 = 70 

b) 23 x 2 – 1 = 23 

c) 250 : 5 + 5 = 25 

d) 40 + 80 : 4 = 30 

e) 80 + 4 : 2 = 42 

f) 70 + 30 : 2 = 50 

g) 90 + 30 x 3 = 180 

h) 70 + 30 : 2 = 85 

\(x^2-4=2\left(x-2\right)\left(x+3\right)\)

\(\Leftrightarrow x^2-4=2\left(x^2+3x-2x-6\right)\)

\(\Leftrightarrow x^2-4=2x^2+2x-12\)

\(\Leftrightarrow x^2-2x^2-2x=-12+4\)

\(\Leftrightarrow-x^2-2x=-8\)

\(\Leftrightarrow-x^2-2x+8=0\)

\(\Leftrightarrow-x^2+2x-4x+8=0\)

\(\Leftrightarrow-x\left(x-2\right)-4\left(x-2\right)=0\)

\(\Leftrightarrow\left(-x-4\right)\left(x-2\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}-x-4=0\\x-2=0\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=-4\\x=2\end{matrix}\right.\)

Vậy \(S=\left\{-4;2\right\}\)

\(x^2-4=2\left(x-2\right)\left(x+3\right)\)

\(\Leftrightarrow\left(x+2\right)\left(x-2\right)=2\left(x-2\right)\left(x+3\right)\)

\(\Leftrightarrow\left(x+2\right)\left(x-2\right)-2\left(x-2\right)\left(x+3\right)=0\)

\(\Leftrightarrow\left(x-2\right)\left[\left(x+2\right)-2\left(x+3\right)\right]=0\)

\(\Leftrightarrow\left(x-2\right)\left(x+2-2x-6\right)=0\)

\(\Leftrightarrow\left(x-2\right)\left(-x-4\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x=2\\x=-4\end{matrix}\right.\)

Đáp án : B

Đáp án: B