x=1/2 a +1,y=1/2 a +2,z=a-3,求x^2+2xy+y^2-2xz+z^2-2yz的值,

x=1/2 a +1,y=1/2 a +2,z=a-3,求x^2+2xy+y^2-2xz+z^2-2yz的值,

由题得 y=x+1 z=2x-5
x^2+2x(x+1)+(x+1)^2-2x(2x-5)+(2x-5)^2-2(x+1)(2x-5)
=x^2+2x^2+2x+x^2+2x+1-4x^2+10x+4x^2-20x+25-4x^2+6x+10
=36

x^2+2xy+y^2-2xz+z^2-2yz
=(x+y-z)^2
然后代入即可。