`@` `\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 \(x=22\)

7 . 3^x + 20 . 3^x = 3²⁵

=> 3^x ( 7+20) = 3²⁵

=> 3^x . 27 = 3²⁵

=> 3^25-x = 27 = 3³

=> 25 - x = 3

=> x = 25 - 3 = 22

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

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

\(\Rightarrow3^x.27=3^{25}\)

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

\(\Rightarrow3^{x+3}=3^{25}\Rightarrow x+3=25\Rightarrow x=22\)

x = 22

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

7x1x7=49

7x2x7=98

7x3x7=147

7x4x7=196

7x5x7=245

7x6x7=294

7x7x7=343

7x8x7=392

7x9x7=441

7x10x7=490

7 x 1 = 7

7 x 2 = 14

7 x 3 = 21

7 x 4 = 28

7 x 5 = 35

7 x 6 = 42

7 x 7 = 49

7 x 8 = 56

7 x 9 = 63

7 x 10 = 70.

7 x 3 = 21

7 x 8 = 56

7 x 2 = 14

7 x 1 = 7

7 x 4 = 35

7 x 6 = 42

7 x 10 = 70

0 x 7 = 0

7 x 7 = 49

7 x 4 = 28

7 x 9 = 63

7 x 0 = 0

7x3=21

7x8=56

7x2=14

7x1=7

7x5=35

7x6=42

7x10=70

0x7=0

7x7=49

7x4=28

7x9=63

7x0=0

7 x 9 = 63       7 x 2 = 14       7 x 5 = 35       7 x 0 = 0

7 x 8 = 56       7 x 3 = 21       7 x 6 = 42       7 x 1 = 7

7 x 7 = 49       7 x 4 = 28       7 x 10 =70       1 x 7 = 7

7 x 1 = 7

7 x 8 = 56

7 x 6 = 42

7 x 5 = 35

7x 2 = 14

7 x 9 = 63

7 x 4 = 28

0 x 7 = 0

7x 3 = 21

7 x 7 = 49

7 x 0 = 0

7 x 10 = 70

7 x 2 = 14       7 x 5 = 35       7 x 6 = 42       0 x 7 = 0

7 x 4 = 28       7 x 3 = 21       7 x 9 = 63       7 x 0 = 0

7 x 8 = 56       7 x 1 = 7       7 x 10 =70       1 x 7 = 7

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