Tìm x: a) 3^x+1 = 9^7 b) 625/5^x = 5^3 c) (-2)^x/-128 = 4

Đáp án:

Giải thích các bước giải:

$\text{a) $3^{x+1}$ = $9^{7}$.}$

$\text{⇒ $3^{x+1}$ = $3^{14}$.}$

$\text{⇒ x + 1 = 14.}$

$\text{⇒ x = 14 – 1.}$

$\text{⇒ x = 13.}$

$\text{b) $\dfrac{625}{5^x}$ = $5^3$.}$

$\text{⇒ $\dfrac{5^4}{5^x}$ = $5^3$.}$

$\text{⇒ $5^{4-x}$ = $5^3$.}$

$\text{⇒ 4 – x = 3.}$

$\text{⇒ x = 4 – 3.}$

$\text{⇒ x = 1.}$

$\text{c) $\dfrac{(-2)^x}{-128}$ = 4.}$

$\text{⇒ $\dfrac{(-2)^x}{(-2)^7}$ = $2^2$.}$

$\text{⇒ $(-2)^{x-7}$ = $(-2)^2$.}$

$\text{⇒ x – 7 = 2.}$

$\text{⇒ x = 2 + 7.}$

$\text{⇒ x =9.}$