原式=(x^2+x)-(4y^2-2y)
=(x^2+x+1/4-1/4)-(4y^2-2y+1/4-1/4)
=[(x+1/2)^2-1/4]-[(2y-1/2)^2-1/4]
=(x+1/2)^2-1/4-(2y-1/2)^2+1/4
=(x+1/2)^2-(2y-1/2)^2
=(x+1/2+2y-1/2)(x+1/2-2y+1/2)
=(x+2y)(x-2y+1)