Bài 4 :
$a,\frac{1}{x}+\frac{5y}{x}=\frac{5y+1}{x}$
$b,\frac{xy}{x+1}+\frac{y}{1+x}=\frac{xy}{x+1}+\frac{y}{x+1}=\frac{xy+y}{x+1}=\frac{y(x+1)}{x+1}$
$c,\frac{1}{(x-1)x}+\frac{1}{x}=\frac{1}{(x-1)x}+\frac{(x-1)}{(x-1)x}=\frac{1+x-1}{(x-1)x}=\frac{x}{x(x-1)}=\frac{1}{x-1}$
Bài 5
$1)3x+5=0$
$3x=-5$
$x=\frac{-5}{3}$
$2)8-4x=0$
$4x=8$
$x=2$
$3)3x+2=5x-10$
$3x-5x=-10-2$
$-2x=-12$
$x=6$