$1)x^2-10xy+25y^2$
$=x^2-2.5xy+(5y)^2$
$=(x-5y)^2$
Vậy ta chọn $B.Sai$
$2)(x-2)^2-(x-3)^2+(x+4)^2$
$=x^2-4x+4-(x^2-6x+9)+x^2+8x+16$
$=x^2-4x+4-x^2+6x-9+x^2+8x+16$
$=(x^2-x^2+x^2)+(-4x+6x+8x)+(4-9+16)$
$=x^2+10x+11$
Vậy ta chọn $A$
$3)A=(x-y+z)^2+(y-x)^2+2(x-y+z)(y-x)$
$=x^2+y^2+z^2-2xy+2xz-2yz+y^2-2xy+x^2+2(xy-x^2-y^2+xy+zy-xz)$
$=x^2+y^2+z^2-2xy+2xz-2yz+y^2-2xy+x^2+2xy-2x^2-2y^2+2xy+2yz-2xz$
$=(x^2+x^2-2x^2)+(y^2+y^2-2y^2)+z^2+(-2xy-2xy+2xy+2xy)+(2xz-2xz)+(-2yz+2yz)$
$=z^2$
Vậy ta chọn $D$