(x+y+z)(x-y-z)-(x+y-z)(x-y+z)
=[x+(y+z)][x-(y+z)]-[x+(y-z)][x-(y-z)]
=X^2 -(Y+Z)^2 -X^2 +(Y-Z)^2
=-Y^2 -2YZ-Z^2 +Y^2 -2YZ+Z^2
=-4YZ
楼上的大哥，第二部就出现了错误了
（x+y+z)(x-y-z)-(x+y-z)(x-y+z)
=[x+(y-z)][x-(y-z)]-[x-(y+z)][x+(y+z)]
=x^2-(y-z)^2-[x^2-(y+z)^2]
=x^2-(y-z)^2-x^2+(y+z)^2
=-(y^2-2yz+z^2)+y^2+2yz+z^2
=4yz
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