$с^2 - 0,25 = 0$
(c − 0,5)(c + 0,5) = 0
$c_1 - 0,5 = 0$
$c_1 = 0,5$;
$c_2 + 0,5 = 0$
$c_2 = -0,5$.

$81x^2 - 121 = 0$
(9x − 11)(9x + 11) = 0
$9x_1 - 11 = 0$
$9x_1 = 11$
$x_1 = \frac{11}{9} = 1\frac{2}{9}$;
$9x_2 + 11 = 0$
$9x_2 = -11$
$x_2 = -\frac{11}{9} = -1\frac{2}{9}$.

$-0,09 + 4x^2 = 0$
$4x^2 - 0,09 = 0$
(2x − 0,3)(2x + 0,3) = 0
$2x_1 - 0,3 = 0$
$2x_1 = 0,3$
$x_1 = \frac{0,3}{2}$
$x_1 = 1,5$;
$2x_2 + 0,3 = 0$
$2x_2 = -0,3$
$x_2 = -\frac{0,3}{2}$
$x_2 = -1,5$.