-12(x-5)+7(3-x)=5

=> -12x+60+21-7x=5

=> -19x+81=5

=>-19x=5-81

=>-19x=-76

=>x=4

Vậy x=4

a: =>1/3*4+1/4*5+...+1/x(x+1)=10/39

=>1/3-1/4+...+1/x-1/x+1=10/39

=>1/3-1/(x+1)=10/39

=>1/(x+1)=13/39-10/39=3/39=1/13

=>x+1=13

=>x=12

b: =>15x=150

=>x=10

c: =>120-5x=45

=>5x=75

=>x=15

 mình cảm ơn ạ

bài này cần có đkxđ 

a, => -12+x-9 = 0

=> x-21=0

=> x=21

b, => -32-x+5 = 0

=> -x-27 = 0

=> -x=27

=> x=-27

c, => 11+15-x = 1

=> 26-x=1

=> x = 26-1 = 25

Tk mk nha

-12 + (x -9)=0              -32 - (x -5)=0                  11 + (15- x )= 1

x -9 =0 +12                        x -5 =0-32                         15 - x =1 - 11

x -9 =12                              x -5  = -32                        15 -x    =-10

x   = 12+9                           x    = ( -32)+5                         x    = 15 +10

x     =2                                x = -27                                    x  =25

a)\(\frac{x^2}{16}=\frac{24}{25}\Rightarrow x^2=\frac{16.24}{25}=\frac{384}{25}\)

\(\Rightarrow x=\frac{8\sqrt{6}}{25}\)hoặc     \(x=-\frac{8\sqrt{6}}{25}\)

b)\(\frac{x}{y}=\frac{9}{10}\Leftrightarrow\frac{x}{9}=\frac{y}{10}=\frac{y-x}{10-9}=\frac{120}{1}=120\)

\(\Rightarrow x=120.9=1080\)và   \(y=120.10=1200\)

c)\(\frac{x}{3}=\frac{y}{5}=\frac{x+y}{3+5}=-\frac{32}{8}=-4\)

\(\Rightarrow x=-4.3=-12\)và     \(y=-4.5=-20\)

d)\(4x=5y\Rightarrow\frac{x}{5}=\frac{y}{4}\Rightarrow\frac{2x}{10}=\frac{y}{4}=\frac{y-2x}{4-10}=\frac{-5}{-6}=\frac{5}{6}\)

\(\Rightarrow x=\frac{5}{6}.5=\frac{25}{6}\)và     \(y=\frac{5}{6}.4=\frac{10}{3}\)

a) \(\frac{x^2}{16}=\frac{24}{25}\)

\(x^2=\frac{24}{25}\cdot16\)

\(x^2=\frac{384}{25}\)

\(x=\sqrt{\frac{384}{25}}=\frac{8\sqrt{6}}{5}\)

Vậy \(x=\frac{8\sqrt{6}}{5}\)

b) \(\frac{x}{y}=\frac{9}{10}\Rightarrow\frac{y}{10}=\frac{x}{9}\)

Áp dụng t/c của dãy tỉ số bằng nhau:

\(\frac{y}{10}=\frac{x}{9}=\frac{y-x}{10-9}=120\)

\(\Rightarrow y=120\cdot10=1200\)

\(x=120\cdot9=1080\)

Vậy y= 1200 , x= 1080 

c) Áp dụng tính chất của dãy tỉ số bằng nhau:

\(\frac{x}{3}=\frac{y}{5}=\frac{x+y}{3+5}=\frac{-32}{8}=-4\)

\(\Rightarrow x=-4\cdot3=-12\)

\(y=-4\cdot5=-20\)

Vậy x=-12 và y= -20

d) \(4x=5y\Rightarrow\frac{x}{5}=\frac{y}{4}\Rightarrow\frac{y}{4}=\frac{2x}{10}\)

Áp dụng t/c của dãy tỉ số bằng nhau:

\(\frac{y}{4}=\frac{2x}{10}=\frac{y-2x}{4-10}=\frac{-5}{-6}=\frac{5}{6}\)

\(\Rightarrow y=\frac{5}{6}\cdot4=\frac{10}{3}\)

\(x=\frac{5}{6}\cdot5=\frac{25}{6}\)

Vậy y= 10/3 và x=25/6 

`7xx8xx12/6xx21xx16`

`=7xx8xx2xxx21xx16`

`=56xx2xx21xx16`

`=112xx21xx16`

`=2352xx16`

`=37632`

\(\dfrac{7\times8\times12}{6\times21\times16}=\dfrac{7\times8\times3\times2\times2}{2\times3\times3\times7\times8\times2}=\dfrac{1}{3}\)

a: \(\frac{23}{45}\cdot\frac{15}{16}\cdot\frac{32}{23}+0,3\cdot x=5\frac35\cdot7\frac{7}{12}\cdot\frac{2}{27}\cdot\frac{5}{13}\)

=>\(\frac{12}{16}\cdot\frac{15}{45}+0,3x=\frac{28}{5}\cdot\frac{91}{12}\cdot\frac{2}{27}\cdot\frac{5}{13}\)

=>\(0,3x+\frac{1}{12}=\frac{91}{13}\cdot\frac{2}{12}\cdot\frac{28}{27}=7\cdot\frac16\cdot\frac{28}{27}=\frac{7\cdot14}{3\cdot27}=\frac{98}{81}\)

=>\(0,3x=\frac{98}{81}-\frac{1}{12}=\frac{365}{324}\)

=>\(x=\frac{365}{324}:0,3=\frac{365}{324\cdot0,3}=\frac{365}{97,2}=\frac{1825}{486}\)

b: \(\left\lbrack\left(35\frac57+2\frac34\right)-5\frac57+\frac14\right\rbrack:11+x=3\)

=>\(\left\lbrack35+\frac57+2+\frac34-5-\frac57+\frac14\right\rbrack:11+x=3\)

=>(32+1):11+x=3

=>33/11+x=3

=>x+3=3

=>x=0