12) 720 : [41 - (2x - 5)] = 2³ . 5

720 : [41 - (2x - 5)] = 8 . 5

720 : [41 - (2x - 5)] = 40

41 - (2x - 5) = 720 : 40

41 - (2x - 5) = 18

2x - 5 = 41 - 18

2x - 5 = 23

2x = 23 + 5

2x = 28

x = 28 : 2

x = 14

Vậy x = 14

14) (5x + 3^4) . 6 ^8 = 6^9 . 3^4

5x + 81 = 6^9 : 6^8 . 3^4

5x + 81 = 6 . 81

5x + 81 = 486

5x = 486 - 81

5x = 405

x = 405 : 5

x = 81

Vậy x = 81

15) (19x + 2 . 5²) : 14 = (13 - 8)² - 4²

(19x + 2 . 25) : 14 = 5² - 16

(19x + 2 . 25) : 14 = 25 - 16

(19x + 2 . 25) : 14 = 9

(19x + 2 . 25) = 9 . 14

19x + 2 . 25 = 126

19x + 50 = 126

19x = 126 - 50

19x = 76

x = 76 : 19

x = 4

Vậy x = 4

17) 130 + [2^4 + (x - 7)] = 215

16 + (x - 7) = 215 - 130

16 + (x - 7) = 85

x - 7 = 85 - 16

x - 7 = 69

x = 69 + 7

x = 76

Vậy x = 76

19) sai đề bài

20) (x - 2) . (x - 3) = 0

\(\left[ \begin{array}{l}x-2=0\\x-3=0\end{array} \right.\) =>\(\left[ \begin{array}{l}x=0+2\\x=0+3\end{array} \right.\) =>\(\left[ \begin{array}{l}x=2\\x=3\end{array} \right.\) 

Vậy x = 2 hoặc  x = 3

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