320

a)
$\left(x^2+1\right)^2=100$
$\sqrt{\left(x^2+1\right)^2}=\sqrt{100}$
$x^2+1=10$
$x^2=9$
$\sqrt{x^2}=\sqrt{9}$
$x=3$
b)
$\left(x^2-3x\right)^2=\left(x^2-2\right)^2\ \ \ \mid\sqrt{ }$
$x^2-3x=x^2-2\ \mid-x^2$
$-3x=-2\ \mid:\left(-3\right)$
$x=\frac{2}{3}$