2/x=3/y=4/z变形得x=1/2zy=3/4z代入：xy-yz+xz=81/2z*(3/4z)-3/4z^2+1/2z^2=8z^2=64代入(x+y)(y-z）(1/2z)^2-(3/4z)^2=1/4z^2-9/16z^2=-5/16*64=-20代入（x^2-xy+y^2)=(1/2z)^2-(1/2z*3/4Z)+(3/4z)^2=1/4z^2-3/8z^2+...
