解方程组4x-3y-6z=0x+2y-7z=0得：x=3z,y=2z(2x^2+3y^2+6z^2)/(x^2+5y^2+7z^2)=[2(3z)^2+3(2z)^2+6z^2]/[(3z)^2+5(2z)^2+7z^2]=[18z^2+12z^2+6z^2]/[9z^2+20z^2+7z^2]=36z^2/36z^2=1
