39 + (31 - x) = 70

31 - x = 70 - 39

31 - x = 31

x = 31 - 31

x = 0

39 + ( 31 - x ) = 70

31 - x              = 70 - 39

31 - x              = 31

x                      = 31 - 31

x                      = 0 .

TL

a)    5x + 20 = 110      

<=> x = 90 : 5 = 18

 b) x + 18 = - 13 

 <=> x = - 31

  c) 120 - x =  50   

   <=> x = 70     

 d) 10 - x = -29  

     <=>  x = 39

e) - x + 31 = 61

<=> x = -30

f) -85 - x = --70

<=> x = 15

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a) 39 + ( 31 - x ) = 70

31 - x = 70 - 39

31 - x = 31

x = 31 - 31

x = 0

b) K đủ dữ liệu

c) 7( x - 9 ) = 35

x - 9 = 35 : 7

x - 9 = 5

x = 5 + 9

x = 14

d) 42 - ( 2x + 32 ) + 12 : 2 = 6

42 - ( 2x + 32 ) + 6 = 6

42 - ( 2x + 32 ) = 6 - 6

42 - ( 2x + 32 ) = 0

2x + 32 = 42 - 0

2x + 32 = 42

2x = 42 - 32

2x = 10

x = 10 : 2

x = 5

tự làm cho quen e ạ

a, 7^x.49=7⁹⁰

<=> 7^x+2=7⁹⁰

<=> x+2=90

<=>x=88

b,x²⁰⁰⁸=x⁵

<=>x²⁰⁰⁸-x⁵=0

<=>x⁵(x²⁰⁰³-1)=0

\(\Leftrightarrow\orbr{\begin{cases}x=0\\x^{2003}-1=0\end{cases}}\Leftrightarrow\orbr{\begin{cases}x=0\\x=1\end{cases}}\)

c,39+(31-x)=70

<=>70-x=70

<=>x=0

d,2.x-26=3²

,=>2x=9+26=35

<=>x=35/2

a/ = 39 x ( 39 + 41 + 20 )

= 39 x 100 = 3900

b/ = 428 x ( 75 + 5 - 70 )

= 428 x 10 = 4280

a) 39 x 39 + 41 x 39 + 39 x 20

= 39 x (39 + 41 + 20)

= 39 x 100

= 3900

b) 428 x 75 + 5 x 428 - 70 x 428

= 428 x (75 + 5 - 70)

= 428 x 10

= 4280

31x72 - 31x70 - 31 x 2 - 31

=31x(72 -70 -2 -1)

=31 x (-1)

= -31

những câu sau thì lam tương tự nha bạn ^_^

\(\Rightarrow\)X*(3/5+2/7)=31/70

\(\Rightarrow\)X*31/35=31/70

\(\Rightarrow\)X=31/70:31/35

\(\Rightarrow\)X=1/2

\(x.\frac{3}{5}+x.\frac{2}{7}=\frac{31}{70}\)

\(x.\frac{31}{35}=\frac{31}{70}\)

\(x=\frac{1}{2}\)

X x 3/5+ Xx 3/7=31/70

=>X x (3/5+3/7)=31/70

=>X x 36/35=31/70

=>X=31/70:36/35

=>X=31/72

X x 3/5+ Xx 3/7=31/70

=>X x (3/5+3/7)=31/70

=>X x 36/35=31/70

=>X=31/70:36/35

=>X=31/72

    39 x31+ 39x68 +39

=39x(31+68+1)

=39x100

=3900

39x3x(31+68)

=117 x (31+68)

117 x 99

= 11583