x+4y-3z=0 (1)4x-5y+2z=0 (2)(1)*4-(2)得 21y=14z y=2z/3代入（1）得 x=z/3(3x²+2xy+z²)/(x²+y²)=z²(1/3+4/9+1)/[z²(1/9+4/9)]=16/5
