1+2xy-x²-y²
=1-(x²-2xy+y²)
=1-(x-y)²
=(1+x-y)(1-x+y)
15x²y-30xy²+3x-6y
=15xy(x-2y)+3(x-2y)
=3(x-2y)(5xy+1)
ax²+ay²-2ay-a³
=a(x²+y²-2ay-a²)
x²-xy+yz-yz
=x²-xy
=x(x-y)