a) (x + 7) – 25 = 13
x+7=13+25
x+7=38
x=38-7
x=31
b) 87 – (73 - x) = 20
73-x=87-20
73-x=67
x=73-67
x=6
c) x - 105: 21 = 15
x-5=15
x=15+5
x=20
d) 15 + (x + 2)²: 3 = 18
(x + 2)²: 3=18-15
(x + 2)²: 3=3
(x + 2)²=3^2
x+2=3
x=1
e) 20 – 2(x – 1)² = 2
2(x-1)^2=20-2
2(x-1)^2=18
(x-1)^2=9
(x-1)^2=3^2
x-1=3
x=4
g, 3^x-1 + 3^x + 3^x+1 =39
3^x-1.(1+3+3^2)=39
3^x-1 . 13=39
3^x-1 =3
x-1=1
x=2

(x + 7) - 25 = 13
x + 7 = 13 + 25
x + 7 = 38
      x = 38 - 7
      x = 31

(X+7)-25=13
(X+7)      =13+25
(X+7)      =38
 X            =38-7
X             =31
Vậy x =31

a) (x + 7) – 25 = 13
<=> x + 7 = 13 + 25
<=> x  = 38 - 7 = 31
b) 87 – (73 - x) = 20
<=>  87 - 73 + x = 20
<=> 14 + x = 20
<=> x = 6
c) x - 105: 21 = 15
<=> x - 5 = 15
<=> x = 15 +5 = 20
d) 15 + (x + 2)²: 3 = 18
<=> (x + 2)² : 3 = 18 -15
<=> (x + 2)² = 3 . 3
<=> (x + 2)² - 3^2 = 0
<=> (x +2 +3)(x+2 -3) = 0 
<=> x = -5
       x = 1
e) 20 - 2( x-1)^2 = 2
<=> 20 -2 - 2(x-1)^2 = 0
<=> 18 -  2(x-1)^2 = 0
<=> 2 . [ 9 - (x-1)^2] = 0
<=> 2 . [ 3^2  - (x-1)^2] = 0
<=> 3^2 - (x-1)^2 = 0
<=> ( 3- x +1)(3+ x-1) = 0
<=> x = 4
       x = -2

a)( x + 7 ) - 25 = 13
     x + 7           = 13 + 25
     x + 7           = 38
     x                 = 38 - 7
     x                 = 31
b) 87 - ( 73 - x ) = 20
              73 - x   = 87 - 20
              73 - x   = 67
                     x    = 73 - 67
                     x    = 6
c) x - 105 : 21 = 15
  ( x - 105 ):21 = 15
    x - 105        = 15 x 21
    x - 105        = 315
    x                 = 315 + 105
    x                 = 420

a) (x+7)=25=31
x+7=13+25
x+7= 38
x+7=38-7
x=31