toan nay la toan lop 4 ma

\(\frac{a}{b+c}=\frac{b}{a+c}=\frac{c}{a+b}=\frac{a+b+c}{b+c+a+c+a+b}=\frac{a+b+c}{2a+2b+2c}=\frac{a+b+c}{2\left(a+b+c\right)}=\frac{1}{2}\)

\(\Rightarrow b+c=2a\)

\(\Rightarrow a+c=2b\)

\(\Rightarrow a+b=2c\)

\(D=\frac{b+c}{a}+\frac{a+c}{b}+\frac{a+b}{c}\)

\(D=\frac{2a}{a}+\frac{2b}{b}+\frac{2c}{c}\)

\(D=2+2+2\)

\(D=6\)

Vậy \(D=6\)