[tex]\frac{x}{y+z+t}=\frac{y}{x+z+t}=\frac{z}{y+x+t}=\frac{y}{y+z+x}=\frac{x+y+z+t}{3(x+y+z+t)}=\frac{1}{3}[/tex]
Suy ra [tex]\left\{\begin{matrix} 3x=y+z+t & \\ 3y=x+z+t & \\ 3z=x+y+t & \\ 3t=x+y+z & \end{matrix}\right.\Rightarrow x=y=z=t[/tex]
Suy ra A=4