a,ĐK:x≥0,x$\neq$ 1
P=($\frac{√x-1}{√x+3}$ -1):($\frac{1}{√x+3}$ +$\frac{√x}{√x-1}$ -$\frac{4√x}{(√x+3)(√x-1)}$
=($\frac{√x-1}{√x+3}$-$\frac{√x+3}{√x+3}$ ):($\frac{√x-1}{(√x+3)(√x-1)}$ +$\frac{√x(√x+3)}{(√x+3)-(√x-1)}$ -$\frac{4√x}{(√x+3)(√x-1)}$
=$\frac{2}{√x+3}$ :($\frac{√x-1+x+3√x-4√x}{(√x+3)(√x-1)}$ )
=$\frac{2}{√x+3}$:$\frac{x-1}{(√x+3)(√x-1)}$
=$\frac{2}{√x+3}$ .$\frac{(√x+3)(√x-1)}{(√x-1)(√x+1)}$
=$\frac{2}{√x+1}$
