`@` `\text {Ans}`

`\downarrow`

`a)`

`210 \div x - 1/2 = 20,5`

`=> 210 \div x = 20,5 + 1/2`

`=> 210 \div x =21`

`=> x = 210 \div 21`

`=> x = 10`

Vậy, `x = 10.`

`b)`

`7 * 3^x + 20*3^x = 3^25`

`=> 3^x * (7+20) = 3^25`

`=> 3^x * 27 = 3^25`

`=> 3^x * 3^3 = 3^25`

`=> 3^x = 3^25 \div 3^3`

`=> 3^x = 3^22`

`=> x = 22`

Vậy, `x = 22.`

a) \(210:x-\dfrac{1}{2}=20,5\)

\(\Rightarrow210:x=20,5+\dfrac{1}{2}\)

\(\Rightarrow210:x=21\)

\(\Rightarrow x=\dfrac{210}{21}\)

\(\Rightarrow x=10\)

b) \(7\cdot3^x+20\cdot3^x=3^{25}\)

\(\Rightarrow3^x\cdot\left(7+20\right)=3^{25}\)

\(\Rightarrow3^x\cdot27=3^{25}\)

\(\Rightarrow3^x\cdot3^3=3^{25}\)

\(\Rightarrow3^{x+3}=3^{25}\)

\(\Rightarrow x+3=25\)

\(\Rightarrow x=25-3\)

\(\Rightarrow x=22\)

\(a)\)

\(210:x-\dfrac{1}{2}=20,5\)

\(210:x=20,5+\dfrac{1}{2}\)

\(210:x=21\)

\(x=210:21\)

\(x=10\)

Vậy \(x=10\)

\(b)\)

\(7.3^x+20.3^x=3^{25}\)

\(3^x.\left(7+20\right)=3^{25}\)

\(3^x.27=3^{25}\)

\(3^x.3^3=3^{25}\)

\(3^x=3^{25}:3^3\)

\(3^x=3^{22}\)

\(x=22\)

Vậy \(x=22\)

=> x-(x+7)=20.3x

20.3x=-7

3x=-7/20

x=-7/60

\(\dfrac{2}{3}\cdot3^{x+1}-7\cdot3^x=-405\)

\(\Rightarrow3^x\cdot\left(\dfrac{2}{3}\cdot3-7\right)=-405\)

\(\Rightarrow3^x\cdot\left(2-7\right)=-405\)

\(\Rightarrow3^x\cdot-5=-405\)

\(\Rightarrow3^x=-405:-5\)

\(\Rightarrow3^x=81\)

\(\Rightarrow3^x=3^4\)

\(\Rightarrow x=4\)

Vậy: \(x=4\)

\(\Leftrightarrow3\cdot\dfrac{2}{3}\cdot3^x-7\cdot3^x=-405\)

=>\(-5\cdot3^x=-405\)

=>3^x=81

=>x=4

A. 3

\(7.3^x=189\)

\(\Rightarrow3^x=27\)

\(\Rightarrow3^x=3^3\Rightarrow x=3\)

Chọn A

Chọn A

Đáp án A

Phương trình  9 x + 1 − 20.3 x + 8 = 0 ⇔ 9 3 x 2 − 20.3 x + 8 = 0 *

Đặt t = 3 x > 0 ,  khi đó * ⇔ 9 t 2 − 20. t + 8 = 0 ⇒ t 1 t 2 = 8 9 ⇔ 3 x 1 .3 x 2 = 8 9 ⇔   x 1 + x 2 = log 3 8 9

x=-216/917

x=-216/917