原式=【（2x+y)-(x-2y)][(2x+y)^2+(2x+y)(x-2y)+(x-2y)^2]
=(x+3y)(4x^2+4xy+y^2+2x^2-3xy-2y^2+x^2-4xy+4y^2)
=(x+3y)(7x^2-3xy+3y^2)