a)

x + 2/5 = 1/2

x          = 1/2 - 2/5

x          = 1/10

b)

x + 3/7 = 2/5 + 3/10

x + 3/7 = 7/10

x         =  7/10 - 3/7

x         = 19/70

c)

19/20 - x = 8/5 - 3/4

19/20 - x = 17/20

x             = 19/20 - 17/20

x             = 1/5

a) x + 2/5 = 1/2 
    x          = 1/2 - 2/5 
    x          = 3/10
b) x + 3/7 = 2/5 + 3/10
    x + 3/7 = 7/10
    x          = 7/10 - 3/7
    x          = 19/70
c) 19/20 - x = 8/5 - 3/4
    19/20 - x = 17/20
                x = 19/20 - 17/20
                x = 2/20 = 1/10

a/ 25^x+1.125²=625⁴

<=> (5²)^x+1.(5³)²=(5⁴)⁴

<=> 5^2x+2.5⁶ = 5¹⁶

<=> 5^2x+2=5¹⁰

=> 2x+2=10 => x=4

b/ 9^x=27³.3⁵

<=> 3^2x=3⁹.3⁵=3¹⁴

=> 2x=14  => x=7

c/ 16^x.8⁵=64⁷

<=> (2⁴)^x.(2³)⁵=(2⁶)⁷

<=> 2^4x.2¹⁵=2⁴²

<=> 2^4x=2²⁷

=> 4x=27 => x=27/4

\(\frac{19}{20}-x+\frac{3}{4}=\frac{8}{5}\)

\(\frac{19}{20}-x=\frac{8}{5}-\frac{3}{4}\)

\(\frac{19}{20}-x=\frac{17}{20}\)

\(x=\frac{19}{20}-\frac{17}{20}\)

 \(x=\frac{1}{10}\)

\(\frac{19}{20}-x+\frac{3}{4}=\frac{8}{5}\)

\(1,7-x=1,6\)

\(x=1,7-1,6\)

\(x=0,1\)

a/\(\dfrac{19}{20}-y=\dfrac{8}{5}-\dfrac{3}{4}\)
\(\dfrac{19}{20}-y=\dfrac{17}{20}\)
\(y=\dfrac{19}{20}-\dfrac{17}{20}\)
\(y=\dfrac{2}{20}=\dfrac{1}{10}\)
b/\(y:\dfrac{2}{3}=\dfrac{4}{9}\times3\)
\(y:\dfrac{2}{3}=\dfrac{4}{3}\)
\(y=\dfrac{4}{3}\times\dfrac{2}{3}\)
\(y=\dfrac{8}{9}\)

#データネ

a) 19/20 - y = 8/5 - 3/4

    19/20 - y =   17/20

                y = 17/20 + 19/20

                y =      36/20

b) y : 2/3 = 4/9 x 3

     y : 2/3 = 12/9

          y    =  12/9 x 2/3 

          y    =   24/27

tích cho mik nhaaaaaaaaaaa

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3: 

\(=\dfrac{1}{7}\cdot\dfrac{3}{5}\cdot\dfrac{5}{6}\cdot\dfrac{5}{8}=\dfrac{1}{7}\cdot\dfrac{1}{2}\cdot\dfrac{5}{8}=\dfrac{5}{112}\)

4:

=>2/3:x=-2-1/3=-7/3

=>x=-2/3:7/3=-2/7

5:

AC=CB=12/2=6cm

IB=6/2=3cm

Bài 13: 

a: =>20-x=15-8+13=20

hay x=0

\(2x+3=8\)

\(\Rightarrow2x=8-3\)

\(\Rightarrow2x=5\)

\(\Rightarrow x=\dfrac{5}{2}\)

\(x:5-2=3\)

\(\Rightarrow x:5=3+2\)

\(\Rightarrow x:5=5\)

\(\Rightarrow x=5\cdot5\)

\(\Rightarrow x=25\)

\(x:7-2=19\)

\(\Rightarrow x:7=19+2\)

\(\Rightarrow x:7=21\)

\(\Rightarrow x=21\cdot7\)

\(\Rightarrow x=147\)

Mình chưa rõ đề

\(20-\left(x+3\right)=5\)

\(\Rightarrow-x-3=5-20\)

\(\Rightarrow-x-3=-15\)

\(\Rightarrow-x=-15+3\)

\(\Rightarrow-x=-12\)

\(\Rightarrow x=12\)

Bài 1 :

\(a)x=\frac{7}{25}+\left(-\frac{1}{5}\right)\)

    \(x=\frac{2}{25}\)

\(b)x=\frac{5}{11}+\left(\frac{4}{-9}\right)\)

    \(x=\frac{1}{99}\)

Mấy câu kia dễ tự làm :>