a)$\frac{x}{-3}$=$\frac{7}{0,75}$

⇒x.0,75=(−3).7

⇒x=$\frac{(-3).7}{0,75}$=−28

Vậy x = 28

b)−0,52:x=$\sqrt{1,96}$:(−1,5)

$\frac{-0,52}{x}$=$\frac{\sqrt{1,96}}{-1,5}$

(−0,52).(−1,5)=x.$\sqrt{1,96}$

0,78 = x. $\sqrt{1,4^2}$

0,78 = x.1,4

1,4.x = 0,78

x=$\frac{78}{140}$

x=$\frac{39}{70}$

Vậy x = $\frac{39}{70}$

c)x:$\sqrt{5}$=$\sqrt{5}$:x

⇔$\frac{x}{\sqrt{5}}$=$\frac{\sqrt{5}}{x}$

⇒x.x=$\sqrt{5}$.$\sqrt{5}$

⇔$x^2$=5

⇔ x= $\sqrt{5}$ hoặc x= $-\sqrt{5}$

Vậy x ∈{$\sqrt{5}$;$-\sqrt{5}$}

Chú ý:

Nếu $x^2$= a (a>0) thì x = $\sqrt{a}$ hoặc x =$-\sqrt{a}$