a) (x - 5).(-10) = -70

x - 5 = -70 : (-10)

x - 5 = 7

x = 7 + 5

x = 12

b, (+3).(+9).10

= 27.10

= 270

c,(-5).7.2

= (-5.2).7

= -10.7

= -70

d, (-12).13.(-5)

= [-12.(-5)].13

= 60.13

= 780

e, (-150).(-4)

= 150.4

= 600

a) 10 - x : 2 = 1

x: 2 = 9

x = 18

b) x:4 + 3 x 2 = 7

x:4 + 6 = 7

x:4 = 1

x = 4

c) 2: x + 5 = 6

2: x = 1

x = 2

+4.10-x:2=1

40-x:2=1

     x:2=40-1

     x:2=39

    x:2=19,5

+5.x:4+3*2=7

5.x:4+6=7

5.x:4=7-6=1

5.x=1.4=4

x=4:5

x=0,8

+6.2:x+5=6

12:x=6-5

12:x=1

x=12:1

x=12

+7.2*x*3*5=30

14.x.15=30

14.x=2

x=2:14

x=1/7

+8.2*x:10*2=1

16.x:20=1

16.x=20

x=20:16

x=1,25

9.18:x-5*2=8

10.5*5-2:x=24

d) 2 * x * 3 * 5 = 30

x * 30 = 30

x = 1

e) 2 * x : 10* 2 = 1

2*x * 1/10 * 2 = 1

2/5 * x = 1

x = 5/2

g) 18 : x - 5*2 = 8

18 : x - 10 = 8

18 : x = 18

x = 1

h) 5*5 -2 : x = 24

25 - 2:x = 24

2:x = 1

x = 2

1: =>3^x=81

=>x=4

2: =>2^x=8

=>x=3

3: =>x^3=2^3

=>x=2

4: =>x^20-x=0

=>x(x^19-1)=0

=>x=0 hoặc x=1

5: =>2^x=32

=>x=5

6: =>(2x+1)^3=9^3

=>2x+1=9

=>2x=8

=>x=4

7: =>x^3=115

=>\(x=\sqrt[3]{115}\)

8: =>(2x-15)^5-(2x-15)^3=0

=>(2x-15)^3*[(2x-15)^2-1]=0

=>2x-15=0 hoặc (2x-15)^2-1=0

=>2x-15=0 hoặc 2x-15=1 hoặc 2x-15=-1

=>x=15/2 hoặc x=8 hoặc x=7

1. Tìm số tự nhiên x biết:

1) \(3^x.3=243\)

\(3^x=243:3\)

\(3^x=81\)

\(3^x=3^4\)

\(\Rightarrow x=4\)

_____

2) \(7.2^x=56\)

\(2^x=56:7\)

\(2^x=8\)

\(2^x=2^3\)

\(\Rightarrow x=3\)

_____

3) \(x^3=8\)

\(x^3=2^3\)

\(\Rightarrow x=3\)

_____

4) \(x^{20}=x\)

\(x^{20}-x=0\)

\(x\left(x^{19}-1\right)=0\)

\(\Rightarrow x=0\) hoặc \(x=1\)

5) \(2^x-15=17\)

\(2^x=17+15\)

\(2^x=32\)

\(2^x=2^5\)

\(\Rightarrow x=5\)

_____

6) \(\left(2x+1\right)^3=9.81\)

\(\left(2x+1\right)^3=729=9^3\)

\(\rightarrow2x+1=9\)

\(2x=9-1\)

\(2x=8\)

\(x=8:2\)

\(\Rightarrow x=4\)

_____

7) \(x^6:x^3=125\)

\(x^3=125\)

\(x^3=5^3\)

\(\Rightarrow x=5\)

_____

8) \(\left(2x-15\right)^5=\left(2x-15\right)^3\)

\(\rightarrow\left(2x-15\right)^5-\left(2x-15\right)^3=0\)

\(\left(2x-15\right)^3.\left[\left(2x-15\right)^2-1\right]=0\)

\(\Leftrightarrow\left\{{}\begin{matrix}\left(2x-15\right)^3=0\\\left(2x-15\right)^2-1=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{15}{2}\\x=7\\x=8\end{matrix}\right.\)

_____

9) \(3^{x+2}-5.3^x=36\)

\(3^x.\left(3^2-5\right)=36\)

\(3^x.\left(9-5\right)=36\)

\(3^x.4=36\)

\(3^x=36:4\)

\(3^x=9\)

\(3^x=3^2\)

\(\Rightarrow x=2\)

_____

10) \(7.4^{x-1}+4^{x+1}=23\)

\(\rightarrow7.4^{x-1}+4^{x-1}.4^2=23\)

\(4^{x-1}.\left(7+4^2\right)=23\)

\(4^{x-1}.\left(7+16\right)=23\)

\(4^{x-1}.23=23\)

\(4^{x-1}=23:23\)

\(4^{x-1}=1\)

\(4^{x-1}=4^1\)

\(\rightarrow x-1=0\)

\(x=0+1\)

\(\Rightarrow x=1\)

Chúc bạn học tốt

\(x+\frac{20}{100}+x=7,2\)

\(< =>2x=7,2-\frac{20}{100}=7,2-0,2\)

\(< =>2x=7< =>x=\frac{7}{2}=3,5\)

x+20%*x=7,2

=> x.100%-20%.x=7,2

=>x.(100%-20%)=7,2

=>x.80%  =7,2

=> x=7,2:80%

=> x=9

x + 20%.x = 72 

<=> 120% .x = 7,2

x = 7,2 : 120 . 100 = 6

Ta có : x + 20% * x = 7,2

            x + 1/5 * x  = 7,2

            x( 1/5 +1)   = 7,2

            x * 6/5        = 7,2

                     x        =  6

\(x\div6\times7,2+1,3\times x+x\div2+15=19,95\)

\(\Leftrightarrow x\times\frac{1}{6}\times7,2+1,3\times x+x\times\frac{1}{2}=19,95-15\)

\(\Leftrightarrow x\times1,2+x\times1,3+x\times\frac{1}{2}=4,95\)

\(\Leftrightarrow x\times\left(1,2+1,3+\frac{1}{2}\right)=4,95\)

\(\Leftrightarrow x\times3=4,95\)

\(\Leftrightarrow x=4,95\div3\)

\(\Leftrightarrow x=1,65\)

Vậy x = 1,65