a.

D: đk 2x +4$\neq$ 0

⇔x+2 $\neq$ 0

⇔x$\neq$  -2

E: đk (x + 2)(x - 2) $\neq$ 0

⇔x$\neq$  ± 2

F: đk (x - 2)($x^{2}$ +2x + 4) $\neq$ 0

⇔x $\neq$ 2(vì $x^{2}$ +2x + 4 luôn lớn hơn 0)

b.

D=$\frac{x^{2} +4X +4 }{2x + 4}$ =$\frac{(x + 2)^{2}}{2(x + 2)}$ =$\frac{x +2}{2}$ 

E=$\frac{-x^{2} + 2x}{(x +2)(x-2)}$  = $\frac{-x(x- 2)}{(x +2)(x-2)}$  =$\frac{-x}{x +2}$

F= $\frac{3(x^{2} +2x + 4) }{(x +2 )( x^{2} +2x + 4)}$ = $\frac{3}{x +2}$ 

c. 

D = 0

⇔ $\frac{x +2}{2}$  =0

⇔x+2=0

⇔x =-2

E = 0

⇔ $\frac{-x}{x +2}$ = 0

⇔x = 0

F = 0

⇔$\frac{3}{x +2}$ = 0

⇔x = 2