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
问题描述：
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
答
第一个对
第二个不对
(a+b-c)^2
=[(a+b)-c]^2
=(a+b)^2-2(a+b)c+c^2
=a^2+b^2+2ab-2ac-2bc+c^2