Bạn thấy :$ \dfrac{2}{a+2}=1-\dfrac{a}{a+2}$
TT $\rightarrow 2( \dfrac{1}{a+2}+ \dfrac{1}{b+2}+ \dfrac{1}{c+2}) =3-( \dfrac{a}{a+2}+ \dfrac{b}{b+2}+ \dfrac{c}{c+2})$
$\leftrightarrow 2.1=3-( \dfrac{a}{a+2}+ \dfrac{b}{b+2}+ \dfrac{c}{c+2})$
$\rightarrow \dfrac{a}{a+2}+ \dfrac{b}{b+2}+ \dfrac{c}{c+2}=1$