=( 1/2){(x-y)^2+x^2-4x+4-4+y^2+2y+5 }

=( 1/2){(x-y)^2+(x-2)^2+y^2+2y+ 1 }

=( 1/2){(x-y)^2+(x-2)^2+(y+ 1)^2 } & gt； =0

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