$B=\sqrt{\frac{x^4-2x^2+1}{x^2}}-\sqrt{\frac{x^4+2x^2+1}{x^2}}=\sqrt{\frac{(x^2-1)^2}{x^2}}-\sqrt{\frac{(x^2+1)^2}{x^2}}$
$=|\frac{x^2-1}{x}|-|\frac{x^2+1}{x}|$
Th1: $x<-1$ khi đó $B=\frac{x^2-1}{-x}-\frac{x^2+1}{-x}=\frac{-2}{-x}=\frac{2}{x}$
Th2: $-1\leq x<0$ khi đó $B=\frac{-x^2+1}{-x}-\frac{x^2+1}{-x}=\frac{-2x^2}{-x}=2x$
Th3: $0\leq x<1$ khi đó $B=\frac{-x^2+1}{x}-\frac{x^2+1}{x}=\frac{-2x^2}{x}=-2x$
Th4: $x\geq 1$ khi đó $B=\frac{x^2-1}{x}-\frac{x^2+1}{x}=\frac{-2}{x}$