a/ $\dfrac{8}{x²+4x}+\dfrac{5}{x+4}-\dfrac{2}{x}$ ($x\ne 0;-4$)
$=\dfrac{8}{x(x+4)}+\dfrac{5x}{x(x+4)}-\dfrac{2(x+4)}{x(x+4)}$
$=\dfrac{8+5x-2x-8}{x(x+4)}$
$=\dfrac{3x}{x(x+4)}=\dfrac{3}{x+4}$
b/ $x=\dfrac{1}{2}$ (TM)
$→P=\dfrac{3}{\dfrac{1}{2}+4}=\dfrac{3}{\dfrac{9}{2}}=\dfrac{2}{3}$