a) x2 - 4x + y2 - 6y + 13
= ( x2 - 4x + 4 ) + ( y2 - 6y + 9 )
= ( x - 2 )2 + ( y - 3 )2
b) 2x2 + y2 + 2xy - 6x - 2y + 5
= ( x2 + 2xy + y2 - 2x - 2y + 1 ) + ( x2 - 4x + 4 )
= [ ( x2 + 2xy + y2 ) - ( 2x + 2y ) + 1 ] + ( x - 2 )2
= [ ( x + y )2 - 2( x + y ) + 12 ] + ( x - 2 )2
= ( x + y - 1 )2 + ( x - 2 )2
c) x2 + 2y2 - 2xy + 8y - 4x + 8
= ( x2 - 2xy + y2 - 4x + 4y + 4 ) + ( y2 + 4y + 4 )
= [ ( x2 - 2xy + y2 ) - 2( x - y )2 + 22 ] + ( y + 2 )2
= [ ( x - y )2 - 2( x - y )2 + 22 ] + ( y + 2 )2
= ( x - y - 2 )2 + ( y + 2 )2