3(y-z)^2-(2y+z)(-z+2y)这道题我是这样做的:3(y-z)^2-(2y+z)(-z+2y)=3(y-z)^2+(-z-2y)(-z+2y)=3(y-z)^2+z^2-4y^2=3(y^2-2yz+z^2)+z^2-4y^2=3y^2-6yz+3z^2+z^2-4y^2=4z^2-6yz-y^2(a+b-c)^2=[(a+b)-c]^2=(a+b)^2-2(a+b)c+c^2=a^2+b^2-2a+2bc+c^2
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