x^3+y^3+3xy-1
=(x+y)^3-3x^2y-3xy^2+3xy-1
=(x+y)^3-3xy(x+y-1)-1
=[(x+y)-1][(x+y)^2+(x+y)+1]-3xy(x+y-1)
=(x+y-1)(x^2+2xy+y^2+x+y+1-3xy)
=(x+y-1)(x^2-xy+y^2+x+y+1)