(1)x²-4x
=x(x-4)
(2)25x²+10x+1
=(5x+1)²
(3)3-9x²
=-3(3x²-1)
(4)1/4[(x-y)²-(x+y)²]
=1/4(x-y+x+y)(x-y-x-y)
=1/4(2x)(-2y)
=-xy