x^2+y^2-6x+10y+34=0
(x^2-6x+9)+(y^2+10y+25)=0
(x-3)^2+(y+5)^2=0
x-3=0,y+5=0
x=3,y=-5
x+y=3-5=-2
2)
a^2+b^2+c^2-ab-ac-bc=0
同时乘以2
a^2+a^2+b^2+b^2+c^2+c^2-2ab-2ac-2bc=0
(a^2-2ab+b^2)+(a^2-2ac+c^2)+(b^2-2bc+c^2)=0
(a-b)^2+(b-c)^2+(a-c)^2=0
a=b=c
等边三角形
2x^2-3x-6xy+9y
=2x^2-6xy-3x+9y
=2x(x-3y)-3(x-3y)
=(2x-3)(x-3y)