Question

B1 Tìm x biết 

 

a) 3^2x=81

b) 5^2x-3-2.5²=5².3

Answer 1

a) 3^2x=81

Mà: 3⁴=81

=>2x=4

=>x=4:2

=>x=2

b) 5^2x-3-2.5²=5².3

=> 5^2x-3-50=75

=>5^2x-3=75+50

=>5^2x-3=125

Mà: 5³=125

=>2x-3=3

=>2x=3+3

=>2x=6

=>x=6:2

=>x=3

Answer 2

a) 3^2x = 81

=> 3^2x = 3⁴

=> 2x = 4

=> x = 2

b) 5^2x-3 - 2.5² = 5².3

5^2x-3 - 50 = 75

=> 5^2x-3 = 125

=> 5^2x-3 = 5³

=> 2x-3 = 3 

=> x = 3

Answer 3

\(3^{2x}=81\)

\(3^{2x}=3^4\)

\(\Rightarrow2x=4\)

     \(x=4:2=2\)

Vậy x=2

Answer 4

a)Ta có:

\(3^{2x}\)= 81

\(\Leftrightarrow\)\(3^{2x}\)=\(3^4\)

\(\Leftrightarrow\)\(3^{2x}\)=\(3^{2\cdot2}\)\(\Rightarrow\)2x=2.2\(\Rightarrow\)x=2.

b)Ta có:

\(5^{2x-3}\)-\(2\cdot5^2\)=\(5^2\cdot3\)

\(\Leftrightarrow5^{2x-3}=5^2\cdot3+2\cdot5^2\)

\(\Leftrightarrow5^{2x-3}=25\cdot3+2\cdot25\)

\(\Leftrightarrow5^{2x-3}=75+50\)

\(\Leftrightarrow5^{2x-3}=125\)

\(\Leftrightarrow5^{2x-3}=5^3\)\(\Rightarrow2x-3=3\Rightarrow x=3\)