1) 31-3.(x+2) = 7

          3.(x+2) = 31- 7

          3.(x+2)  =  24

              x+2   = 24:3

               x + 2  =  8

               x       = 8-2

              x           = 6

2) 175-25.(2x-1) = 175

            25.(2x-1)  =  175- 175 = 0

                     2x-1  =  0 : 25 = 0

                     2x   = 0 + 1 = 1

                       x    =  1 : 2 = 0,5

3) 25 - x/2 = 19

           x/2  =  25-19

          x/2   =    6

         x : 2   = 6

         x          = 6 x 2 = 12

4) 2.(x-1)+3 = 15

    2.(x-1)        =  15 - 3 = 12

         x - 1        =  12 : 2 = 6

        x               =  6+1 = 7

a) `123 - 27 + 27 + 13 - 123`

`= (123 - 123) + (27 - 27) + 13`

`= 0 + 0 + 13`

`= 13`

`---`

b) `175 + 25 + 13 + 15 - 175 - 25`

`= (175-175)+(25-25)+25+13`

`=0+0+25+13`

`=28`

`---`

c) \(53\times39+47\times39+53\times21+47\times21\)

\(=\text{[}\left(53+47\right)\times39]+\left[\left(53+47\right)\times21\right]\)

\(=\left(100\times39\right)+\left(100\times21\right)\)

\(=3900+2100\)

`=6000`

`---`

d) \(49\times35+49\times16-49\times61\)

`=` \(49\times\left(35+16-61\right)\)

`=` \(49\times\left(-10\right)\)

`=-490`

Ta có: \(\frac{x+1}{25}+\frac{x+3}{23}+\frac{x+5}{21}+\frac{x+7}{19}=-4\)

=>\(\left(\frac{x+1}{25}+1\right)+\left(\frac{x+3}{23}+1\right)+\left(\frac{x+5}{21}+1\right)+\left(\frac{x+7}{19}+1\right)=-4+4=0\)

=>\(\frac{x+26}{25}+\frac{x+26}{23}+\frac{x+26}{21}+\frac{x+26}{19}=0\)

=>\(\left(x+26\right)\left(\frac{1}{25}+\frac{1}{23}+\frac{1}{21}+\frac{1}{19}\right)=0\)

=>x+26=0

=>x=-26

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

\(\Rightarrow\orbr{\begin{cases}x=0\\x-2=0\end{cases}}\)

\(\Rightarrow\orbr{\begin{cases}x=0\\x=2\end{cases}}\)

a) \(\dfrac{7}{9}\times\dfrac{13}{25}\times\dfrac{9}{7}=\left(\dfrac{7}{9}\times\dfrac{9}{7}\right)\times\dfrac{13}{25}=1\times\dfrac{13}{25}=\dfrac{13}{25}\)

b) \(\dfrac{5}{9}\times\dfrac{21}{25}+\dfrac{21}{25}\times\dfrac{4}{9}\)

\(=\dfrac{21}{25}\times\left(\dfrac{5}{9}+\dfrac{4}{9}\right)\)

\(=\dfrac{21}{25}\times1=\dfrac{21}{25}\)

c) \(\dfrac{19}{5}\times\dfrac{11}{16}-\dfrac{9}{5}\times\dfrac{11}{16}\)

\(=\dfrac{11}{16}\times\left(\dfrac{19}{5}-\dfrac{9}{5}\right)\)

\(=\dfrac{11}{16}\times2=\dfrac{22}{16}=\dfrac{11}{8}\)

a: \(=\dfrac{18}{51}\cdot\dfrac{34}{9}\cdot\dfrac{17}{18}=\dfrac{17}{51}\cdot\dfrac{34}{9}=\dfrac{1}{3}\cdot\dfrac{34}{9}=\dfrac{34}{27}\)

b: \(=\dfrac{21}{32}\cdot\dfrac{4}{7}\cdot25\cdot\dfrac{4}{5}=\dfrac{21}{7}\cdot\dfrac{4}{32}\cdot20=60\cdot\dfrac{1}{8}=\dfrac{15}{2}\)

=24/21+150/175

<=>8/7+6/7=14/7=2

Đúng rồi đó, bằng 2.
Ta có: 7/21+3/21+14/21=7+3+14/21=24/21
125/175+25/175=125+25/175=150/175
Vì 24/21=8/7;150/175=6/7 nên 8/7+6/7=14/7=2
 

bằng 2 chứ sao trăng gì nữa