Ta có: [tex]\frac{x}{y+z}+\frac{y}{z+x}+\frac{z}{x+y}=1[/tex]
[tex]\Rightarrow\frac{x(x+y+z)}{y+z}+\frac{y(x+y+z)}{z+x}+\frac{z(x+y+z)}{x+y}=x+y+z[/tex]
[tex]\Rightarrow\frac{x^{2}+x(y+z)}{y+z}+\frac{y^{2}+y(x+z)}{z+x}+\frac{z^{2}(x+y)}{x+y}=x+y+z[/tex]
[tex]\Rightarrow\frac{x^{2}}{y+z}+\frac{y^{2}}{z+x}+\frac{z^{2}}{x+y}+x+y+z=x+y+z[/tex]
[tex]\Rightarrow\frac{x^{2}}{y+z}+\frac{y^{2}}{z+x}+\frac{z^{2}}{x+y}=0[/tex]
Thay vào A, tính được A=9