a) x^50=x b)125=x^3 c) 64=x^2 d) 90=10.3^x

$a$) $x^{50} = x$

$⇔ x^{50} – x = 0$

$⇔ x.(x^{49} – 1) =0$

$⇒$ \(\left[ \begin{array}{l}x=0\\x^{49}=1⇔ x=1\end{array} \right.\) 

    Vậy $x$ $∈$ `{0;1}`.

$b$) $125 = x^3$

$⇔ 5^3 = x^3$

$⇔ x=5$

   Vậy $x=5$.

$c$) $64 = x^2$

$⇔ (±8)^2 = x^2$

$⇔ x ∈$ `{±8}`

   Vậy $x$ $∈$ `{±8}`.

$d$) $90 = 10.3^x$

$⇔ 3^x = 9$

$⇔ 3^x = 3^2$

$⇔ x=2$

   Vậy $x=2$.