`x : 5/2 = 7/3`

`=> x= 7/3 xx 5/2`

`=>x= 35/6`

__

`23,7 xx 49 +23,7 xx 51`

`= 23,7 xx (49+51)`

`= 23,7 xx 100`

`=2370`

Lời giải:
a. 

$x=\frac{7}{3}\times \frac{5}{2}=\frac{35}{6}$

b.

$23,7\times 49+23,7\times 51=23,7\times (49+51)=23,7\times 100=2370$

x: 5/2=7/3`

`=> x = 7/3 xx 5/2`

`=> x=35/6`

Vậy `x = 35/6`

`23,7 xx 49 + 23,7 xx 51`

` =23,7 xx(49+51)`

`= 23,7 xx 100`

`=2370`

\(x-43=35-x-48\)

\(x+x=43+35-48=30\)

2.x=30=> x=15

2) -x+6-85=x+51-54

-x-79=x-3

2x=76=> x=38

3)\(\orbr{\begin{cases}-7-x-3=-3\Rightarrow x=-7\\-7-x-3=3\Rightarrow x=-13\end{cases}}\)

\(a.3x^2-50=142\)

\(3x^2=142+50\)

\(3x^2=192\)

\(x^2=192:3\)

\(x^2=64\)

\(\Rightarrow x=8\)

vậy: x=8.

\(b.3x^2=192\)

\(x^2=192:3\)

\(x^2=64\)

\(\Rightarrow x=8\)

Vậy: x=8.

\(c.4x^2=100\)

\(x^2=100:4\)

\(x^2=25\)

\(\Rightarrow x=5\)

vậy: x=5.

\(d.x^2+7=56\)

\(x^2=56-7\)

\(x^2=49\)

\(\Rightarrow x=7\)

Vậy: x=7.

<câu e bạn xem lại cho mk có phải sai đầu bài không nha,tại mk thấy kết quả là 21 nhưng mà không có cái nào bình phương =21 cả> 

$a.3{}^{}-50=142$

$3{}^{}=142+50$

$3{}^{}=192$

${}^{}=192:3$

${}^{}=64$

$\Rightarrow x=8$

vậy: x=8.

$b.3{}^{}=192$

${}^{}=192:3$

${}^{}=64$

$\Rightarrow x=8$

Vậy: x=8.

$c.4{}^{}=100$

${}^{}=100:4$

${}^{}=25$

$\Rightarrow x=5$

\(2\frac{4}{5}.x-50:\frac{2}{3}=51\)

\(\frac{14}{5}.x-50:\frac{2}{3}=51\)

\(\frac{14}{51}.x=51+50:\frac{2}{3}\)

\(\frac{14}{51}.x=51+75\)

\(\frac{14}{51}.x=126\)

\(x=126:\frac{14}{51}\)

x=459

Vậy x=459

\(\frac{2}{3}.x=\frac{5}{12}=>x=\frac{5}{12}:\frac{2}{3}=\frac{5.3}{12.2}=\frac{15}{24}\)=\(\frac{3}{8}\)

vậy x=3/8

\(-\frac{2}{3}.x+\frac{1}{5}=\frac{3}{10}nên-\frac{2}{3}.x=\frac{3}{10}-\frac{1}{5}\)

\(-\frac{2}{3}x=\frac{1}{10}nênx=\frac{1}{10}:-\frac{2}{3}=-\frac{3}{20}\)

Vậy x=-3/20

\(a,\dfrac{-1}{8}=\dfrac{3}{x}\\ \dfrac{3}{-24}=\dfrac{3}{x}\\ x=-24\\ b,\dfrac{x}{3}=\dfrac{3}{x}\\ x.x=3.3\\ x^2=9\\ x=\pm3\\ c,\dfrac{3}{4}.x=1\dfrac{1}{2}\\ \dfrac{3}{4}.x=\dfrac{3}{2}\\ x=\dfrac{3}{2}:\dfrac{3}{4}\\ x=2\\ d,x-\dfrac{3}{10}=\dfrac{7}{15}:\dfrac{3}{5}\\ x-\dfrac{3}{10}=\dfrac{7}{9}\\ x=\dfrac{7}{9}+\dfrac{3}{10}\\ x=\dfrac{97}{90}\\ e,\dfrac{-4}{7}-x=\dfrac{-8}{3}.\dfrac{3}{7}\\ \dfrac{-4}{7}-x=\dfrac{-8}{7}\\ x=\dfrac{-4}{7}+\dfrac{8}{7}\\ x=\dfrac{4}{7}\\ \)

f: =>x*4/7=1/5+2/3=3/15+10/15=13/15

=>x=91/60

g: =>1/3:3x=1/5-2/3=-7/15

=>3x=-5/7

=>x=-5/21

h: =>7/8x=-1/9

=>x=-1/9:7/8=-1/9*8/7=-8/63

j: =>(2,8x-50)=34

=>2,8x=84

=>x=30