$a$) `[(x.x+95)-31] . 14 = 560`
`⇔(x^2 + 95) - 31 = 40`
$⇔ x^2 + 95 = 71$
$⇔ x^2 = -24$
$⇒ x$ $∈$ $∅$
Vậy $x$ $∈$ $∅$
$b$) $5^{x+1} + 6.5^{x+1} = 875$
$⇔ 5^{x+1} . 7 = 875$
$⇔ 5^{x+1} = 125$
$⇔ 5^{x+1} = 5^3$
$⇔ x = 2$
Vậy $x=2$
$c$) $(21-3)^4 = (2x-3)^4$
$⇔ 18^4 = (2x-3)^4$
$⇒$ \(\left[ \begin{array}{l}2x=21\\2x=-15\end{array} \right.\)
$⇒$ \(\left[ \begin{array}{l}x=\dfrac{21}{2}\\x=\dfrac{-15}{2}\end{array} \right.\)
Vậy $x$ $∈$ `{{21}/2;{-15}/2}`
