(1)-x^2+6x-9因式分解
=-(x^2-6x+9)
=-(x-3)^2
(2)25x^-10x^2+1=(5x-1)^2
;
(3)(m+n)^2-6(m+n)+9=(m+n-3)^2 ;
(4)-4xy-4x^2-y^2
=-(4x^2+4xy+y^2)
=-(2x+y)^2