(3x+y-z)(3x-y+z)
=[3x+(y-z)] * [3x-(y-z)]
=(3x)^2 - (y-z)^2
=9x^2 - (y^2-2yz+z^2)
=9x^2 - y^2+2yz-z^2
(3x+y-z)(3x-y+z)等于多少
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