tìm số tự nhiên x, biết: 1 ) 65 - 5( 2x+6)= 25

256 :[12-(x+3)] = 64

12-(x+3)=256:64

12-(x+3)=4

x+3=12-4

x+3=8

x=5

( 2x - 1)⁴ = 81

(2x-1)⁴=3⁴

2x-1=3

2x=4

x=2

5) 28. 2^x +1 - 22.2^x -1 = 96

6.2^x=96

2^x=16

2^x=2⁴

x=4

6) 2³ .(x-1)³ = 512

8. (x-1)³=512

(x-1)³=64

(x-1)³=4³

x-1=4

x=5

7) 20 chia hết ( x-3) và 42 chia hết cho ( x-3)

=> (x-3) ∈ ƯC(20,42) ={1,2}

x-3=1 =>x=4

x-3=2 => x=5

( x+ 2) + (x+12) + ( x + 42 ) + ( x + 47) =655

4x+103=655

4x=655-103

4x=552

x=552:4=138

x+ ( x+1) +(x+2) +...+ ( x+2008) = 2008.2009

2009x + 2008x(2008+1):2=2008.2009

2009x + 1004.2009=2008.2009

2009x = 1004.2009

x = 1004

11) 1+2+3+...+x=820

x.(x+1):2=820

x.(x+1)=1640

x=40